Basic Engineering Mathematics

Basic Engineering Mathematics In memory of Elizabeth Basic Engineering Mathematics Fourth Edition John Bird, BSc(H...

Author: John Bird BSc (Hons) CEng CMath CSci FIET MIEE FIIE FIMA FCollT

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Basic Engineering Mathematics

In memory of Elizabeth

Basic Engineering Mathematics Fourth Edition

John Bird, BSc(Hons), CMath, CEng, FIMA, MIEE, FIIE(Elec), FCollP

Newnes An imprint of Elsevier Linacre House, Jordan Hill, Oxford OX2 8DP 30 Corporate Drive, Burlington, MA 01803 First published 1999 Second edition 2000 Third edition 2002 Reprinted 2000 (twice), 2001, 2003 Fourth edition 2005 Copyright © 2005, John Bird. All rights reserved. The right of John Bird to be identified as the author of this work has been asserted in accordance with the Copyright, Designs and Patents Act 1988 No part of this publication may be reproduced in any material form (including photocopying or storing in any medium by electronic means and whether or not transiently or incidentally to some other use of this publication) without the written permission of the copyright holder except in accordance with the provisions of the Copyright, Designs and Patents Act 1988 or under the terms of a licence issued by the Copyright Licensing Agency Ltd, 90 Tottenham Court Road, London, England W1T 4LP. Applications for the copyright holder’s written permission to reproduce any part of this publication should be addressed to the publisher Permissions may be sought directly from Elsevier’s Science and Technology Rights Department in Oxford, UK: phone: (+44) (0) 1865 843830; fax: (+44) (0) 1865 853333; e-mail: [email protected]. You may also complete your request on-line via the Elsevier homepage (http://www.elsevier.com), by selecting ‘Customer Support’ and then ‘Obtaining Permissions’ British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library Library of Congress Cataloguing in Publication Data A catalogue record for this book is available from the Library of Congress ISBN 0 7506 6575 0

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Typeset by Charon Tec Pvt. Ltd, Chennai, India www.charontec.com Printed and bound in Great Britain

Contents

Preface

xi

1.

Basic arithmetic

1

1.1 1.2 1.3

1 3 4

2.

Fractions, decimals and percentages 2.1 2.2 2.3 2.4

3.

4.

5.

Arithmetic operations Highest common factors and lowest common multiples Order of precedence and brackets

Fractions Ratio and proportion Decimals Percentages Assignment 1

6 6 8 9 11 13

Indices, standard form and engineering notation

14

3.1 3.2 3.3 3.4 3.5 3.6 3.7

14 14 16 17 18 19 19

Indices Worked problems on indices Further worked problems on indices Standard form Worked problems on standard form Further worked problems on standard form Engineering notation and common prefixes

Calculations and evaluation of formulae

21

4.1 4.2 4.3 4.4

21 22 25 27 29

Errors and approximations Use of calculator Conversion tables and charts Evaluation of formulae Assignment 2

Computer numbering systems

30

5.1 5.2 5.3 5.4 5.5

30 30 31 32 33

Binary numbers Conversion of binary to denary Conversion of denary to binary Conversion of denary to binary via octal Hexadecimal numbers

6. Algebra

37

6.1 6.2 6.3

37 39 41

Basic operations Laws of indices Brackets and factorization

vi

Contents

6.4 6.5

7.

Fundamental laws and precedence Direct and inverse proportionality Assignment 3

Simple equations

47

7.1 7.2 7.3 7.4 7.5

47 47 49 50 52

Expressions, equations and identities Worked problems on simple equations Further worked problems on simple equations Practical problems involving simple equations Further practical problems involving simple equations

8. Transposition of formulae 8.1 8.2 8.3 8.4

9.

43 45 46

Introduction to transposition of formulae Worked problems on transposition of formulae Further worked problems on transposition of formulae Harder worked problems on transposition of formulae Assignment 4

54 54 54 55 57 59

Simultaneous equations

60

9.1 9.2 9.3 9.4 9.5

60 60 62 63 65

Introduction to simultaneous equations Worked problems on simultaneous equations in two unknowns Further worked problems on simultaneous equations More difficult worked problems on simultaneous equations Practical problems involving simultaneous equations

10. Quadratic equations 10.1 10.2 10.3 10.4 10.5 10.6

Introduction to quadratic equations Solution of quadratic equations by factorization Solution of quadratic equations by ‘completing the square’ Solution of quadratic equations by formula Practical problems involving quadratic equations The solution of linear and quadratic equations simultaneously

11. Inequalities 11.1 11.2 11.3 11.4 11.5 11.6

Introduction to inequalities Simple inequalities Inequalities involving a modulus Inequalities involving quotients Inequalities involving square functions Quadratic inequalities Assignment 5

12. Straight line graphs 12.1 Introduction to graphs 12.2 The straight line graph 12.3 Practical problems involving straight line graphs 13. Graphical solution of equations 13.1 Graphical solution of simultaneous equations 13.2 Graphical solutions of quadratic equations

69 69 69 71 72 73 75 77 77 77 78 79 79 80 82 83 83 83 88 94 94 95

Contents

13.3 Graphical solution of linear and quadratic equations simultaneously 13.4 Graphical solution of cubic equations Assignment 6 14. Logarithms 14.1 14.2 14.3 14.4

Introduction to logarithms Laws of logarithms Indicial equations Graphs of logarithmic functions

15. Exponential functions 15.1 15.2 15.3 15.4 15.5 15.6 15.7

The exponential function Evaluating exponential functions The power series for ex Graphs of exponential functions Napierian logarithms Evaluating Napierian logarithms Laws of growth and decay Assignment 7

16. Reduction of non-linear laws to linear-form 16.1 Determination of law 16.2 Determination of law involving logarithms 17. Graphs with logarithmic scales 17.1 17.2 17.3 17.4

Logarithmic scales Graphs of the form y = axn Graphs of the form y = abx Graphs of the form y = aekx

18. Geometry and triangles 18.1 18.2 18.3 18.4 18.5 18.6

Angular measurement Types and properties of angles Properties of triangles Congruent triangles Similar triangles Construction of triangles Assignment 8

19. Introduction to trigonometry 19.1 19.2 19.3 19.4 19.5 19.6

Trigonometry The theorem of Pythagoras Trigonometric ratios of acute angles Solution of right-angled triangles Angles of elevation and depression Evaluating trigonometric ratios of any angles

20. Trigonometric waveforms 20.1 Graphs of trigonometric functions 20.2 Angles of any magnitude 20.3 The production of a sine and cosine wave

vii

99 100 102 103 103 103 105 106 107 107 107 108 110 111 111 113 116 117 117 119 124 124 124 127 128 131 131 132 134 136 137 139 141 142 142 142 143 145 147 148 151 151 152 154

viii

Contents

20.4 Sine and cosine curves 20.5 Sinusoidal form A sin(ωt ± α) Assignment 9 21. Cartesian and polar co-ordinates 21.1 21.2 21.3 21.4

Introduction Changing from Cartesian into polar co-ordinates Changing from polar into Cartesian co-ordinates Use of R → P and P → R functions on calculators

22. Areas of plane figures 22.1 22.2 22.3 22.4 22.5

Mensuration Properties of quadrilaterals Worked problems on areas of plane figures Further worked problems on areas of plane figures Areas of similar shapes Assignment 10

23. The circle 23.1 23.2 23.3 23.4

Introduction Properties of circles Arc length and area of a sector The equation of a circle

24. Volumes of common solids 24.1 24.2 24.3 24.4 24.5

Volumes and surface areas of regular solids Worked problems on volumes and surface areas of regular solids Further worked problems on volumes and surface areas of regular solids Volumes and surface areas of frusta of pyramids and cones Volumes of similar shapes Assignment 11

25. Irregular areas and volumes and mean values of waveforms 25.1 Areas of irregular figures 25.2 Volumes of irregular solids 25.3 The mean or average value of a waveform 26. Triangles and some practical applications 26.1 26.2 26.3 26.4 26.5 26.6

Sine and cosine rules Area of any triangle Worked problems on the solution of triangles and their areas Further worked problems on the solution of triangles and their areas Practical situations involving trigonometry Further practical situations involving trigonometry Assignment 12

27. Vectors 27.1 Introduction 27.2 Vector addition 27.3 Resolution of vectors

155 158 161 162 162 162 163 164 166 166 166 167 171 172 173 174 174 174 175 178 180 180 180 182 186 189 190 191 191 193 194 198 198 198 198 200 201 204 206 207 207 207 209

Contents

27.4 Vector subtraction 27.5 Relative velocity 28. Adding of waveforms 28.1 Combination of two periodic functions 28.2 Plotting periodic functions 28.3 Determining resultant phasors by calculation 29. Number sequences 29.1 29.2 29.3 29.4 29.5 29.6 29.7 29.8

Simple sequences The n’th term of a series Arithmetic progressions Worked problems on arithmetic progression Further worked problems on arithmetic progressions Geometric progressions Worked problems on geometric progressions Further worked problems on geometric progressions Assignment 13

30. Presentation of statistical data 30.1 Some statistical terminology 30.2 Presentation of ungrouped data 30.3 Presentation of grouped data 31. Measures of central tendency and dispersion 31.1 31.2 31.3 31.4 31.5

Measures of central tendency Mean, median and mode for discrete data Mean, median and mode for grouped data Standard deviation Quartiles, deciles and percentiles

32. Probability 32.1 32.2 32.3 32.4

Introduction to probability Laws of probability Worked problems on probability Further worked problems on probability Assignment 14

33. Introduction to differentiation 33.1 33.2 33.3 33.4 33.5 33.6 33.7 33.8 33.9 33.10

Introduction to calculus Functional notation The gradient of a curve Differentiation from first principles Differentiation of y = axn by the general rule Differentiation of sine and cosine functions Differentiation of eax and ln ax Summary of standard derivatives Successive differentiation Rates of change

ix

210 212 214 214 214 215 218 218 218 219 220 221 222 223 224 225 226 226 227 230 235 235 235 236 237 239 241 241 241 242 243 246 247 247 247 248 249 250 252 253 254 255 255

x

Contents

34. Introduction to integration 34.1 34.2 34.3 34.4 34.5

The process of integration The general solution of integrals of the form axn Standard integrals Definite integrals Area under a curve Assignment 15

257 257 257 257 260 261 265

List of formulae

266

Answers to exercises

270

Index

285

Preface

Basic Engineering Mathematics, 4th Edition introduces and then consolidates basic mathematical principles and promotes awareness of mathematical concepts for students needing a broad base for further vocational studies. In this fourth edition, new material has been added on engineering notation, inequalities, graphs with logarithmic scales and adding waveforms, together with extra practical problems interspersed throughout the text. The text covers: (i) the Applied Mathematics content of the GNVQ mandatory unit ‘Applied Science and Mathematics for Engineering’ at Intermediate level (i.e. GNVQ 2) (ii) the mandatory ‘Mathematics for Engineering’ at Advanced level (i.e. GNVQ 3) in Engineering (iii) the optional ‘Applied Mathematics for Engineering’ at Advanced level (i.e. GNVQ 3) in Engineering (iv) the Mathematics content of ‘Applied Science and Mathematics for Technicians’ for Edexcel/BTEC First Certificate (v) the mandatory ‘Mathematics for Technicians’ for National Certificate and National Diploma in Engineering (vi) Mathematics 1 for City & Guilds Technician Certificate in Telecommunications and Electronics Engineering (vii) basic mathematics for a wide range of introductory/access/foundation mathematics courses (viii) GCSE revision, and for similar mathematics courses in English-speaking countries world-wide. Basic Engineering Mathematics 4th Edition provides a lead into Engineering Mathematics 4th Edition. Each topic considered in the text is presented in a way that assumes in the reader little previous knowledge of that topic. Theory is introduced in each chapter by a brief outline of essential theory, definitions, formulae, laws and procedures. However, these are kept to a minimum, for problem solving is extensively used to establish and exemplify the theory. It is intended that readers will gain real understanding through seeing problems solved and then solving similar problems themselves. This textbook contains some 600 worked problems, followed by over 1050 further problems (all with answers – at the end of the book). The further problems are contained within some 129 Exercises; each Exercise follows on directly from the relevant section of work. 260 line diagrams enhance the understanding of the theory. Where at all possible the problems mirror practical situations found in engineering and science. At regular intervals throughout the text are 15 Assignments to check understanding. For example, Assignment 1 covers material contained in chapters 1 and 2, Assignment 2 covers the material contained in chapters 3 and 4, and so on. These Assignments do not have answers given since it is envisaged that lecturers could set the Assignments for students to attempt as part of their course structure. Lecturers may obtain a complimentary set of solutions of the Assignments in an Instructor’s Manual available from the publishers via the internet – see below. At the end of the book a list of relevant formulae contained within the text is included for convenience of reference. ‘Learning by Example’ is at the heart of Basic Engineering Mathematics 4th Edition. John Bird Defence College of Electro-Mechanical Engineering, HMS Sultan, formerly University of Portsmouth and Highbury College, Portsmouth Instructor’s Manual An Instructor’s Manual containing the full worked solutions for all the Assignments in this book is available for download for lecturers only. To obtain a password please e-mail [email protected] with the following details: course title, number of students, your job title and work postal address. To download the Instructor’s Manual use the following direct URL: http://books.elsevier.com/manualsprotected/0750665750

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1 Basic arithmetic

Thus 27 − 74 + 81 − 19 = 15

1.1 Arithmetic operations Whole numbers are called integers. +3, +5, +72 are called positive integers; −13, −6, −51 are called negative integers. Between positive and negative integers is the number 0 which is neither positive nor negative. The four basic arithmetic operators are: add (+), subtract (−), multiply (×) and divide (÷) For addition and subtraction, when unlike signs are together in a calculation, the overall sign is negative. Thus, adding minus 4 to 3 is 3 + −4 and becomes 3 − 4 = −1. Like signs together give an overall positive sign. Thus subtracting minus 4 from 3 is 3 − − 4 and becomes 3 + 4 = 7. For multiplication and division, when the numbers have unlike signs, the answer is negative, but when the numbers have like signs the answer is positive. Thus 3 × − 4 = −12, whereas −3 × −4 = +12. Similarly 4 4 =− −3 3

and

−4 4 =+ −3 3

Problem 1. Add 27, −74, 81 and −19 This problem is written as 27 − 74 + 81 − 19 Adding the positive integers: Sum of positive integers is: Adding the negative integers: Sum of negative integers is:

27 81 108 74 19 93

Taking the sum of the negative integers from the sum of the positive integers gives: 108 −93 15

Problem 2.

Subtract 89 from 123

This is written mathematically as 123 − 89 123 −89 34 Thus 123 − 89 = 34 Problem 3.

Subtract −74 from 377

This problem is written as 377 − − 74. Like signs together give an overall positive sign, hence 377 − −74 = 377 + 74

377 +74 451

Thus 377 − −74 = 451 Problem 4.

Subtract 243 from 126

The problem is 126 − 243. When the second number is larger than the first, take the smaller number from the larger and make the result negative. Thus 126 − 243 = −(243 − 126)

243 −126 117

Thus 126 − 243 = −117 Problem 5.

Subtract 318 from −269

2

Basic Engineering Mathematics

−269 − 318. The sum of the negative integers is 269 +318 587 Thus −269 − 318 = −587

When dividing by numbers which are larger than 12, it is usual to use a method called long division. 27 14 378 28 98 98 00

(2) 2 × 14 → (4) 7 × 14 →

Problem 6.

Multiply 74 by 13

Thus 378 ÷ 14 = 27

This is written as 74 × 13

Adding:

74 13 222 740 962

Problem 10. ← 74 × 3 ← 74 × 10

Thus 74 × 13 = 962 Problem 7.

(1) 14 into 37 goes twice. Put 2 above the 7 of 378. (3) Subtract. Bring down the 8. 14 into 98 goes 7 times. Put 7 above the 8 of 378. (5) Subtract.

Multiply by 178 by −46

When the numbers have different signs, the result will be negative. (With this in mind, the problem can now be solved by multiplying 178 by 46) 178 46 1068 7120 8188

Divide 5669 by 46

This problem may be written as

5669 or 5669 ÷ 46 or 46

5669/46 Using the long division method shown in Problem 9 gives: 123 46 5669 46 106 92 149 138 11 As there are no more digits to bring down, 5669 ÷ 46 = 123, remainder 11

or

123

11 46

Thus 178 × 46 = 8188 and 178 × (−46) = −8188 Now try the following exercise Problem 8.

Divide l043 by 7 Exercise 1

When dividing by the numbers 1 to 12, it is usual to use a method called short division. 149 7 103 46 3 7 into 10 goes 1, remainder 3. Put 1 above the 0 of 1043 and carry the 3 remainder to the next digit on the right, making it 34;

Step 2.

7 into 34 goes 4, remainder 6. Put 4 above the 4 of 1043 and carry the 6 remainder to the next digit on the right, making it 63; 7 into 63 goes 9, remainder 0. Put 9 above the 3 of 1043.

Thus 1043 ÷ 7 = 149 Problem 9.

In Problems 1 to 21, determine the values of the expressions given: 1. 67 − 82 + 34

Step 1.

Step 3.

Further problems on arithmetic operations (Answers on page 270)

Divide 378 by 14

2. 124 − 273 + 481 − 398 3. 927 − 114 + 182 − 183 − 247 4. 2417 − 487 + 2424 − 1778 − 4712 5. −38419 − 2177 + 2440 − 799 + 2834 6. 2715 − 18250 + 11471 − 1509 + 113274 7. 73 − 57 8. 813 − (−674) 9. 647 − 872 10. 3151 − (−2763) 11. 4872 − 4683

Basic arithmetic

factor (HCF) is the largest number which divides into two or more numbers exactly. A multiple is a number which contains another number an exact number of times. The smallest number which is exactly divisible by each of two or more numbers is called the lowest common multiple (LCM).

12. −23148 − 47724 13. 38441 − 53774 14. (a) 261 × 7 15. (a) 783 × 11

(b) 462 × 9 (b) 73 × 24

16. (a) 27 × 38 (b) 77 × 29 17. (a) 448 × 23

3

(b) 143 × (−31)

18. (a) 288 ÷ 6 (b) 979 ÷ 11 896 1813 (b) 19. (a) 7 16 21432 (b) 15904 ÷ 56 20. (a) 47 88738 (b) 46857 ÷ 79 21. (a) 187 22. A screw has a mass of 15 grams. Calculate, in kilograms, the mass of 1200 such screws. 23. Holes are drilled 35.7 mm apart in a metal plate. If a row of 26 holes is drilled, determine the distance, in centimetres, between the centres of the first and last holes. 24. Calculate the diameter d and dimensions A and B for the template shown in Figure 1.1. All dimensions are in millimetres.

Problem 11. Determine the HCF of the numbers 12, 30 and 42 Each number is expressed in terms of its lowest factors. This is achieved by repeatedly dividing by the prime numbers 2, 3, 5, 7, 11, 13 . . . (where possible) in turn. Thus 12 ⫽ 2 ⫻ 2 ⫻ 3 30 ⫽ 2

⫻ 3 ⫻5

42 ⫽ 2

⫻ 3 ⫻7

The factors which are common to each of the numbers are 2 in column 1 and 3 in column 3, shown by the broken lines. Hence the HCF is 2 × 3, i.e. 6. That is, 6 is the largest number which will divide into 12, 30 and 42. Problem 12. Determine the HCF of the numbers 30, 105, 210 and 1155

110 B

Using the method shown in Problem 11: 30 ⫽ 2⫻ 3 ⫻ 5

12

105 ⫽

d

3⫻ 5⫻ 7

210 ⫽ 2⫻ 3 ⫻ 5 ⫻ 7

A

1155 ⫽ 60

3 ⫻ 5 ⫻ 7⫻11

The factors which are common to each of the numbers are 3 in column 2 and 5 in column 3. Hence the HCF is 3 × 5 = 15. 38

50

Problem 13. and 90

Determine the LCM of the numbers 12, 42

120

Fig. 1.1

The LCM is obtained by finding the lowest factors of each of the numbers, as shown in Problems 11 and 12 above, and then selecting the largest group of any of the factors present. Thus 12 ⫽ 2 ⫻ 2 ⫻ 3

1.2 Highest common factors and lowest common multiples When two or more numbers are multiplied together, the individual numbers are called factors. Thus a factor is a number which divides into another number exactly. The highest common

42 ⫽ 2

⫻3

90 ⫽ 2

⫻ 3⫻ 3 ⫻ 5

⫻7

The largest group of any of the factors present are shown by the broken lines and are 2 × 2 in 12, 3 × 3 in 90, 5 in 90 and 7 in 42.

4

Basic Engineering Mathematics

Hence the LCM is 2 × 2 × 3 × 3 × 5 × 7 = 1260, and is the smallest number which 12, 42 and 90 will all divide into exactly.

(ii) 2 × 3 = 3 × 2, i.e. the order of numbers when multiplying does not matter; (iii) 2 + (3 + 4) = (2 + 3) + 4, i.e. the use of brackets when adding does not affect the result;

Problem 14. Determine the LCM of the numbers 150, 210, 735 and 1365

Using the method shown in Problem 13 above:

(vii) 2[3 + (4 × 5)] = 2[3 + 20] = 2 × 23 = 46, i.e. when an expression contains inner and outer brackets, the inner brackets are removed first.

210 ⫽ 2 ⫻ 3 ⫻ 5⫻ 7

1365 ⫽

3 ⫻ 5⫻ 7⫻ 7 3 ⫻ 5⫻ 7

(v) 2 × (3 + 4) = 2(3 + 4) = 2 × 3 + 2 × 4, i.e. a number placed outside of a bracket indicates that the whole contents of the bracket must be multiplied by that number; (vi) (2 + 3)(4 + 5) = (5)(9) = 45, i.e. adjacent brackets indicate multiplication;

150 ⫽ 2 ⫻ 3 ⫻ 5⫻ 5

735 ⫽

(iv) 2 × (3 × 4) = (2 × 3) × 4, i.e. the use of brackets when multiplying does not affect the result;

⫻ 13 Problem 15.

Find the value of 6 + 4 ÷ (5 − 3)

The LCM is 2 × 3 × 5 × 5 × 7 × 7 × 13 = 95550 Now try the following exercise

The order of precedence of operations is remembered by the word BODMAS. Thus

Exercise 2

6 + 4 ÷ (5 − 3) = 6 + 4 ÷ 2

Further problems on highest common factors and lowest common multiples (Answers on page 270)

In Problems 1 to 6 find (a) the HCF and (b) the LCM of the numbers given: 1. 6, 10, 14

2. 12, 30, 45

3. 10, 15, 70, 105

4. 90, 105, 300

5. 196, 210, 910, 462

6. 196, 350, 770

Problem 16.

= 6+2

(Division)

=8

(Addition)

Determine the value of 13 − 2 × 3 + 14 ÷ (2 + 5)

13 − 2 × 3 + 14 ÷ (2 + 5) = 13 − 2 × 3 + 14 ÷ 7

1.3 Order of precedence and brackets When a particular arithmetic operation is to be performed first, the numbers and the operator(s) are placed in brackets. Thus 3 times the result of 6 minus 2 is written as 3 × (6 − 2). In arithmetic operations, the order in which operations are performed are:

Problem 17.

(B)

= 13 − 2 × 3 + 2

(D)

= 13 − 6 + 2

(M)

= 15 − 6

(A)

=9

(S)

Evaluate 16 ÷ (2 + 6) + 18[3 + (4 × 6) − 21]

(i) to determine the values of operations contained in brackets; (ii) multiplication and division (the word ‘of’ also means multiply); and

(Brackets)

16 ÷ (2 + 6) + 18[3 + (4 × 6) − 21]

(iii) addition and subtraction.

= 16 ÷ (2 + 6) + 18[3 + 24 − 21)

(B)

This order of precedence can be remembered by the word BODMAS, standing for Brackets, Of, Division, Multiplication, Addition and Subtraction, taken in that order. The basic laws governing the use of brackets and operators are shown by the following examples:

= 16 ÷ 8 + 18 × 6

(B)

= 2 + 18 × 6

(D)

= 2 + 108

(M)

= 110

(A)

(i) 2 + 3 = 3 + 2, i.e. the order of numbers when adding does not matter;

Basic arithmetic

Now try the following exercise Problem 18.

Find the value of 23 − 4(2 × 7) +

(144 ÷ 4) (14 − 8)

Exercise 3

Further problems on order of precedence and brackets (Answers on page 270)

Simplify the expressions given in Problems 1 to 7: 36 (144 ÷ 4) = 23 − 4 × 14 + 23 − 4(2 × 7) + (14 − 8) 6 = 23 − 4 × 14 + 6

(B) (D)

= 23 − 56 + 6

(M)

= 29 − 56

(A)

= −27

(S)

1. 14 + 3 × 15 2. 17 − 12 ÷ 4 3. 86 + 24 ÷ (14 − 2) 4. 7(23 − 18) ÷ (12 − 5) 5. 63 − 28(14 ÷ 2) + 26 6.

112 − 119 ÷ 17 + (3 × 19) 16

7.

(50 − 14) + 7(16 − 7) − 7 3

5

2 Fractions, decimals and percentages

2.1 Fractions When 2 is divided by 3, it may be written as 23 or 2/3. 23 is called a fraction. The number above the line, i.e. 2, is called the numerator and the number below the line, i.e. 3, is called the denominator. When the value of the numerator is less than the value of the denominator, the fraction is called a proper fraction; thus 23 is a proper fraction. When the value of the numerator is greater than the denominator, the fraction is called an improper fraction. Thus 73 is an improper fraction and can also be expressed as a mixed number, that is, an integer and a proper fraction. Thus the improper fraction 73 is equal to the mixed number 2 13 . When a fraction is simplified by dividing the numerator and denominator by the same number, the process is called cancelling. Cancelling by 0 is not permissible. Problem 1.

Simplify

1 3

+ 27

The LCM of the two denominators is 3 × 7, i.e. 21. Expressing each fraction so that their denominators are 21, gives: 1 2 1 7 2 3 7 6 + = × + × = + 3 7 3 7 7 3 21 21 =

13 7+6 = 21 21

Thus

1 2 7 + 6 13 + = = as obtained previously. 3 7 21 21

Problem 2.

Find the value of 3 23 − 2 16

One method is to split the mixed numbers into integers and their fractional parts. Then 1 2 1 1 2 2 − 2+ =3+ −2− 3 −2 = 3+ 3 6 3 6 3 6 4 1 3 1 = 1+ − =1 =1 6 6 6 2 Another method is to express the mixed numbers as improper fractions. 2 9 2 11 9 Since 3 = , then 3 = + = 3 3 3 3 3 1 12 1 13 + = Similarly, 2 = 6 6 6 6 1 11 13 22 13 9 1 2 − = − = = 1 as obtained Thus 3 − 2 = 3 6 3 6 6 6 6 2 previously.

Alternatively: Step(2)

Step(3)

↓

↓

(7 × 1) + (3 × 2) 1 2 + = 3 7 21 ↑ Step(1)

Step 1: the LCM of the two denominators; Step 2: for the fraction 13 , 3 into 21 goes 7 times, 7 × the numerator is 7 × 1; Step 3: for the fraction 27 , 7 into 21 goes 3 times, 3 × the numerator is 3 × 2.

Problem 3.

Evaluate 7 18 − 5 37

1 3 3 1 7+ − 5+ =7+ −5− 8 7 8 7 1 3 7×1−8×3 = 2+ − =2+ 8 7 56

3 1 7 −5 = 8 7

Fractions, decimals and percentages

= 2+

−17 7 − 24 =2+ 56 56

= 2−

112 17 112 − 17 17 = − = 56 56 56 56

=

Multiplying both numerator and denominator by the reciprocal of the denominator gives: 1 2 1 3 × 3 3 1 7 2 4 = 4 = 3 7 = 1 1 12 1 4 1 2 2 11 × 21 2 1 1 21 1 3

39 95 =1 56 56

Problem 4.

Determine the value of 4 58 − 3 14 + 1 25

1 2 5 4 − 3 + 1 = (4 − 3 + 1) + 8 4 5

5 1 2 − + 8 4 5

5 × 5 − 10 × 1 + 8 × 2 40 25 − 10 + 16 = 2+ 40 31 31 = 2+ =2 40 40

This method can be remembered by the rule: invert the second fraction and change the operation from division to multiplication. Thus: 1 3 3 2 1 3 12 ÷ = × as obtained previously. = 7 21 1 24 4 7 1 3

= 2+

Problem 5.

Find the value of

3 14 × 7 15

Problem 8.

Dividing numerator and denominator by 3 gives:

3 × 14 = 1 × 14 = 1 × 14 7 1 55 7 5 7×5 Dividing numerator and denominator by 7 gives: 1

2 1 42 1×2 1× = = 1×5 5 7×5 1 This process of dividing both the numerator and denominator of a fraction by the same factor(s) is called cancelling. Evaluate 1 35 × 2 13 × 3 37

Mixed numbers must be expressed as improper fractions before multiplication can be performed. Thus, 3 5 3 3 6 1 21 3 1 1 ×2 ×3 = + + + × × 3 7 5 5 3 3 7 7 5 =

8 1 8×1×8 7 2 4 × × = 5 1 5×1×1 3 7 1

=

64 4 = 12 5 5

8

Problem 7.

Simplify

3 3 12 7 ÷ = 12 7 21 21

3 12 ÷ 7 21

Find the value of 5 35 ÷ 7 13

The mixed numbers must be expressed as improper fractions. Thus, 14 3 1 28 22 3 42 2 8 5 ÷7 = ÷ = × = 5 3 5 3 5 2 2 11 55 Problem 9.

Problem 6.

7

Simplify

1 3

−

2 5

+ 14 ÷ 38 × 13

The order of precedence of operations for problems containing fractions is the same as that for integers, i.e. remembered by BODMAS (Brackets, Of, Division, Multiplication, Addition and Subtraction). Thus, 1 2 1 3 1 − + × ÷ 3 5 4 8 3 =

1 4×2+5×1 31 ÷ − 20 3 2 48

13 1 82 × − 2 0 3 5 1 1 26 = − 5 3 (5 × 1) − (3 × 26) = 15 13 −73 = −4 = 15 15

=

(B) (D) (M) (S)

Problem 10. Determine the value of 1 1 3 1 7 1 of 3 − 2 − +5 ÷ 6 2 4 8 16 2 1 1 3 1 7 1 of 3 − 2 − +5 ÷ 6 2 4 8 16 2 =

1 41 3 1 7 of 1 + ÷ − 6 4 8 16 2

(B)

8

Basic Engineering Mathematics

=

3 1 7 5 41 × + ÷ − 6 4 8 16 2

(O)

13.

=

1 7 5 41 1 62 × + − × 6 4 3 2 8 1

(D)

=

35 82 1 + − 24 3 2

14. If a storage tank is holding 450 litres when it is threequarters full, how much will it contain when it is twothirds full?

(M)

35 + 656 1 − = 24 2

(A)

=

691 1 − 24 2

(A)

=

691 − 12 24

(S)

7 679 = 28 = 24 24 Now try the following exercise Exercise 4

Further problems on fractions (Answers on page 270)

Evaluate the expressions given in Problems 1 to 13: 1 2 1. (a) + 2 5

7 1 (b) − 16 4

3 2 2. (a) + 7 11

2 1 2 (b) − + 9 7 3

1 2 ×1 3 4

÷

2 1 + 3 4

+1

3 5

15. Three people, P, Q and R contribute to a fund, P provides 3/5 of the total, Q provides 2/3 of the remainder, and R provides £8. Determine (a) the total of the fund, (b) the contributions of P and Q.

2.2 Ratio and proportion The ratio of one quantity to another is a fraction, and is the number of times one quantity is contained in another quantity of the same kind. If one quantity is directly proportional to another, then as one quantity doubles, the other quantity also doubles. When a quantity is inversely proportional to another, then as one quantity doubles, the other quantity is halved. Problem 11.

Divide 126 in the ratio of 5 to 13.

Because the ratio is to be 5 parts to 13 parts, then the total number of parts is 5 + 13, that is 18. Then, 18 parts correspond to 126 126 = 7, 1 part corresponds to 18

3 3 3. (a) 5 + 3 13 4

5 2 (b) 4 − 3 8 5

Hence

2 3 4. (a) 10 − 8 7 3

1 4 5 (b) 3 − 4 + 1 4 5 6

5 parts correspond to 5 × 7 = 35 and 13 parts correspond to 13 × 7 = 91

17 15 × 35 119

(Check: the parts must add up to the total 35 + 91 = 126 = the total.)

5. (a)

3 5 × 4 9

6. (a)

2 3 7 × ×1 5 9 7

(b)

(b)

3 5 1 ×1 7. (a) × 4 11 39 8. (a) 9.

3 45 ÷ 8 64

13 7 4 ×4 ×3 17 11 39

3 4 (b) ÷ 1 4 5

1 5 (b) 1 ÷ 2 3 9

1 3 16 − × 3 4 27

9 1 1 3 + ÷ − 2 5 15 3 5 7 3 15 of 15 × ÷ 11. + 15 7 4 16 10.

1 2 1 3 2 12. × − ÷ + 4 3 3 5 7

Problem 12. A piece of timber 273 cm long is cut into three pieces in the ratio of 3 to 7 to 11. Determine the lengths of the three pieces. The total number of parts is 3 + 7 + 11, that is, 21. Hence 21 parts correspond to 273 cm

3 parts correspond to

273 = 13 cm 21 3 × 13 = 39 cm

7 parts correspond to

7 × 13 = 91 cm

1 part corresponds to

11 parts correspond to 11 × 13 = 143 cm i.e. the lengths of the three pieces are 39 cm, 91 cm and 143 cm. (Check: 39 + 91 + 143 = 273)

Fractions, decimals and percentages

Problem 13. A gear wheel having 80 teeth is in mesh with a 25 tooth gear. What is the gear ratio? 80 16 = = 3.2 25 5 i.e. gear ratio = 16:5 or 3.2:1 Gear ratio = 80 : 25 =

Problem 14.

Express 25p as a ratio of £4.25 l

Working in quantities of the same kind, the required ratio is 25 425

i.e.

1 17

1 th of £4.25. This may be written either as: 17 25:425: :1:17 (stated as ‘25 is to 425 as 1 is to 17’) or as

9

4. When mixing a quantity of paints, dyes of four different colours are used in the ratio of 7:3:19:5. If the mass of the first dye used is 3 12 g, determine the total mass of the dyes used. 5. Determine how much copper and how much zinc is needed to make a 99 kg brass ingot if they have to be in the proportions copper:zinc: :8:3 by mass. 6. It takes 21 hours for 12 men to resurface a stretch of road. Find how many men it takes to resurface a similar stretch of road in 50 hours 24 minutes, assuming the work rate remains constant. 7. It takes 3 hours 15 minutes to fly from city A to city B at a constant speed. Find how long the journey takes if:

That is, 25p is

(b) if the speed is three-quarters of the original speed.

25 1 = 425 17 Problem 15. An alloy is made up of metals A and B in the ratio 2.5:1 by mass. How much of A has to be added to 6 kg of B to make the alloy?

Ratio A:B: :2.5:1 i.e. When B = 6 kg,

A 2.5 = = 2.5 B 1

A = 2.5 from which, A = 6 × 2.5 = 15 kg 6

Problem 16. If 3 people can complete a task in 4 hours, find how long it will take 5 people to complete the same task, assuming the rate of work remains constant. The more the number of people, the more quickly the task is done, hence inverse proportion exists. 3 people complete the task in 4 hours, 1 person takes three times as long, i.e. 4 × 3 = 12 hours, 5 people can do it in one fifth of the time that one person takes, 12 hours or 2 hours 24 minutes. that is 5 Now try the following exercise Exercise 5

(a) the speed is 1 12 times that of the original speed and

Further problems on ratio and proportion (Answers on page 270)

1. Divide 312 mm in the ratio of 7 to 17.

2.3 Decimals The decimal system of numbers is based on the digits 0 to 9. A number such as 53.17 is called a decimal fraction, a decimal point separating the integer part, i.e. 53, from the fractional part, i.e. 0.17. A number which can be expressed exactly as a decimal fraction is called a terminating decimal and those which cannot be expressed exactly as a decimal fraction are called nonterminating decimals. Thus, 32 = 1.5 is a terminating decimal, but 43 = 1.33333 . . . is a non-terminating decimal. ˙ called ‘one point-three 1.33333 . . . can be written as 1.3, recurring’. The answer to a non-terminating decimal may be expressed in two ways, depending on the accuracy required: (i) correct to a number of significant figures, that is, figures which signify something, and (ii) correct to a number of decimal places, that is, the number of figures after the decimal point. The last digit in the answer is unaltered if the next digit on the right is in the group of numbers 0, 1, 2, 3 or 4, but is increased by 1 if the next digit on the right is in the group of numbers 5, 6, 7, 8 or 9. Thus the non-terminating decimal 7.6183 . . . becomes 7.62, correct to 3 significant figures, since the next digit on the right is 8, which is in the group of numbers 5, 6, 7, 8 or 9. Also 7.6183 . . . becomes 7.618, correct to 3 decimal places, since the next digit on the right is 3, which is in the group of numbers 0, 1, 2, 3 or 4.

2. Divide 621 cm in the ratio of 3 to 7 to 13. 3. £4.94 is to be divided between two people in the ratio of 9 to 17. Determine how much each person will receive.

Problem 17.

Evaluate 42.7 + 3.04 + 8.7 + 0.06

10

Basic Engineering Mathematics

The numbers are written so that the decimal points are under each other. Each column is added, starting from the right. 42.7 3.04 8.7 0.06 54.50

37.81 ÷ 1.7 =

Thus 42.7 + 3.04 + 8.7 + 0.06 = 54.50 Problem 18. Take 81.70 from 87.23 The numbers are written with the decimal points under each other. 87.23 −81.70 5.53 Thus 87.23 − 81.70 = 5.53 Problem 19.

Problem 21. Evaluate 37.81 ÷ 1.7, correct to (i) 4 significant figures and (ii) 4 decimal places.

Find the value of

23.4 − 17.83 − 57.6 + 32.68 The sum of the positive decimal fractions is 23.4 + 32.68 = 56.08 The sum of the negative decimal fractions is 17.83 + 57.6 = 75.43 Taking the sum of the negative decimal fractions from the sum of the positive decimal fractions gives: 56.08 − 75.43

37.81 1.7

The denominator is changed into an integer by multiplying by 10. The numerator is also multiplied by 10 to keep the fraction the same. Thus 37.81 ÷ 1.7 =

378.1 37.81 × 10 = 1.7 × 10 17

The long division is similar to the long division of integers and the first four steps are as shown: 22.24117.. 17 378.100000 34 — 38 34 — 41 34 — 70 68 — 20 (i) 37.81 ÷ 1.7 = 22.24, correct to 4 significant figures, and (ii) 37.81 ÷ 1.7 = 22.2412, correct to 4 decimal places. Problem 22. Convert (a) 0.4375 to a proper fraction and (b) 4.285 to a mixed number.

i.e. −(75.43 − 56.08) = −19.35 Problem 20.

Determine the value of 74.3 × 3.8

When multiplying decimal fractions: (i) the numbers are multiplied as if they are integers, and (ii) the position of the decimal point in the answer is such that there are as many digits to the right of it as the sum of the digits to the right of the decimal points of the two numbers being multiplied together. Thus (i)

743 38 5944 22 290 28 234

(ii) As there are (1 + 1) = 2 digits to the right of the decimal points of the two numbers being multiplied together, (74.3 × 3.8), then 74.3 × 3.8 = 282.34

(a) 0.4375 can be written as its value,

0.4375 × 10 000 without changing 10 000

4375 10 000 875 175 35 7 4375 = = = = By cancelling 10 000 2000 400 80 16 7 i.e. 0.4375 = 16 57 285 =4 (b) Similarly, 4.285 = 4 1000 200 i.e. 0.4375 =

Problem 23. Express as decimal fractions: 9 7 (a) and (b) 5 16 8

(a) To convert a proper fraction to a decimal fraction, the numerator is divided by the denominator. Division by 16 can

Fractions, decimals and percentages

be done by the long division method, or, more simply, by dividing by 2 and then 8: 4.50 2 9.00

0.5 6 2 5 8 4.55 02 04 0

Thus,

9 = 0.5625 16

(b) For mixed numbers, it is only necessary to convert the proper fraction part of the mixed number to a decimal fraction. Thus, dealing with the 78 gives: 0.875 8 7.000

14. 13 15. 8

11

31 , correct to 2 decimal places. 37

9 , correct to 3 significant figures. 13

16. Determine the dimension marked x in the length of shaft shown in Figure 2.1. The dimensions are in millimetres. 82.92

7 i.e. = 0.875 8

27.41

8.32

x

34.67

7 Thus 5 = 5.875 8 Now try the following exercise Exercise 6

Further problems on decimals (Answers on page 270)

In Problems 1 to 7, determine the values of the expressions given:

Fig. 2.1 17. A tank contains 1800 litres of oil. How many tins containing 0.75 litres can be filled from this tank?

1. 23.6 + 14.71 − 18.9 − 7.421 2. 73.84 − 113.247 + 8.21 − 0.068

2.4 Percentages

3. 5.73 × 4.2

Percentages are used to give a common standard and are fractions having the number 100 as their denominators. For 25 1 example, 25 per cent means i.e. and is written 25%. 100 4

4. 3.8 × 4.1 × 0.7 5. 374.1 × 0.006 6. 421.8 ÷ 17, (a) correct to 4 significant figures and (b) correct to 3 decimal places. 7.

0.0147 , (a) correct to 5 decimal places and 2.3 (b) correct to 2 significant figures.

8. Convert to proper fractions: (a) 0.65 (b) 0.84 (c) 0.0125 (d) 0.282 and (e) 0.024

Problem 24. (b) 0.0125

Express as percentages: (a) 1.875 and

A decimal fraction is converted to a percentage by multiplying by 100. Thus, (a) 1.875 corresponds to 1.875 × 100%, i.e. 187.5% (b) 0.0125 corresponds to 0.0125 × 100%, i.e. 1.25%

9. Convert to mixed numbers: (a) 1.82 (b) 4.275 (c) 14.125 (d) 15.35 and (e) 16.2125 In Problems 10 to 15, express as decimal fractions to the accuracy stated: 4 10. , correct to 5 significant figures. 9 17 , correct to 5 decimal place. 11. 27 12. 1

9 , correct to 4 significant figures. 16

13. 53

5 , correct to 3 decimal places. 11

Problem 25. Express as percentages: 5 2 (a) and (b) 1 16 5 To convert fractions to percentages, they are (i) converted to decimal fractions and (ii) multiplied by 100 5 5 = 0.3125, hence corresponds to 16 16 0.3125 × 100%, i.e. 31.25% 2 (b) Similarly, 1 = 1.4 when expressed as a decimal fraction. 5 2 Hence 1 = 1.4 × 100% = 140% 5 (a) By division,

12

Basic Engineering Mathematics

Problem 26. It takes 50 minutes to machine a certain part. Using a new type of tool, the time can be reduced by 15%. Calculate the new time taken. 750 15 × 50 = = 7.5 minutes. 100 100 Hence the new time taken is 50 − 7.5 = 42.5 minutes. 15% of 50 minutes =

Alternatively, if the time is reduced by 15%, then it now 85 × 50 = takes 85% of the original time, i.e. 85% of 50 = 100 4250 = 42.5 minutes, as above. 100 Problem 27.

Find 12.5% of £378

12.5% of £378 means hundred’.

12.5 × 378, since per cent means ‘per 100

Hence 12.5% of £378 =

1 378 12.5 8 × 378 = 8 = £47.25 100

Problem 28. Express 25 minutes as a percentage of 2 hours, correct to the nearest 1%. Working in minute units, 2 hours = 120 minutes. Hence 25 ths of 2 hours. 25 minutes is 120 5 25 = By cancelling, 120 24 5 as a decimal fraction gives 0.2083˙ Expressing 24 Multiplying by 100 to convert the decimal fraction to a percentage gives: ˙ 0.2083˙ × 100 = 20.83%

Thus, the masses of the copper, zinc and nickel are 2.244 kg, 0.935 kg and 0.561 kg, respectively. (Check: 2.244 + 0.935 + 0.561 = 3.74)

Now try the following exercise Exercise 7

Further problems percentages (Answers on page 271)

1. Convert to percentages: (a) 0.057

(b) 0.374

(c) 1.285

2. Express as percentages, correct to 3 significant figures: 7 19 11 (a) (b) (c) 1 33 24 16 3. Calculate correct to 4 significant figures: (a) 18% of 2758 tonnes (b) 47% of 18.42 grams (c) 147% of 14.1 seconds 4. When 1600 bolts are manufactured, 36 are unsatisfactory. Determine the percentage unsatisfactory. 5. Express: (a) 140 kg as a percentage of 1 t (b) 47 s as a percentage of 5 min (c) 13.4 cm as a percentage of 2.5 m 6. A block of monel alloy consists of 70% nickel and 30% copper. If it contains 88.2 g of nickel, determine the mass of copper in the block. 7. A drilling machine should be set to 250 rev/min. The nearest speed available on the machine is 268 rev/min. Calculate the percentage overspeed. 8. Two kilograms of a compound contains 30% of element A, 45% of element B and 25% of element C. Determine the masses of the three elements present.

Thus 25 minutes is 21% of 2 hours, correct to the nearest 1%. Problem 29. A German silver alloy consists of 60% copper, 25% zinc and 15% nickel. Determine the masses of the copper, zinc and nickel in a 3.74 kilogram block of the alloy. By direct proportion: 100% corresponds to 3.74 kg 3.74 = 0.0374 kg 1% corresponds to 100 60% corresponds to 60 × 0.0374 = 2.244 kg 25% corresponds to 25 × 0.0374 = 0.935 kg 15% corresponds to 15 × 0.0374 = 0.561 kg

9. A concrete mixture contains seven parts by volume of ballast, four parts by volume of sand and two parts by volume of cement. Determine the percentage of each of these three constituents correct to the nearest 1% and the mass of cement in a two tonne dry mix, correct to 1 significant figure. 10. In a sample of iron ore, 18% is iron. How much ore is needed to produce 3600 kg of iron? 11. A screws’ dimension is 12.5 ± 8% mm. Calculate the possible maximum and minimum length of the screw. 12. The output power of an engine is 450 kW. If the efficiency of the engine is 75%, determine the power input.

Fractions, decimals and percentages

Assignment 1 This assignment covers the material contained in Chapters 1 and 2. The marks for each question are shown in brackets at the end of each question.

(a) 2016 − (−593) (b) 73 + 35 ÷ (11 − 4) 120 − 133 ÷ 19 + (2 × 17) (c) 15 1 5 2 + (d) − 5 15 6 (e) 49.31 − 97.763 + 9.44 − 0.079 2. Determine, by long multiplication 37 × 42 4675 11

7. Evaluate

40

75

120

576.29 19.3

(a)

correct to 4 significant figures

(b)

correct to 1 decimal place

(4)

(14) 9 correct to 2 decimal places 46

(2)

(3)

8. Express 7

(3)

9. Determine, correct to 1 decimal places, 57% of 17.64 g (2)

4. Find (a) the highest common factor, and (b) the lowest common multiple of the following numbers: 15

(9)

6. A piece of steel, 1.69 m long, is cut into three pieces in the ratio 2 to 5 to 6. Determine, in centimeters, the lengths of the three pieces. (4)

1. Evaluate the following:

3. Evaluate, by long division

1 2 5. Simplify (a) 2 ÷ 3 3 3 1 1 7 1 ÷ + +2 (b) 1 4 3 5 24 ×2 7 4

(6)

10. Express 54.7 mm as a percentage of 1.15 m, correct to 3 significant figures. (3)

13

3 Indices, standard form and engineering notation

3.1 Indices The lowest factors of 2000 are 2 × 2 × 2 × 2 × 5 × 5 × 5. These factors are written as 24 × 53 , where 2 and 5 are called bases and the numbers 4 and 3 are called indices. When an index is an integer it is called a power. Thus, 24 is called ‘two to the power of four’, and has a base of 2 and an index of 4. Similarly, 53 is called ‘five to the power of 3’ and has a base of 5 and an index of 3. Special names may be used when the indices are 2 and 3, these being called ‘squared’and ‘cubed’, respectively. Thus 72 is called ‘seven squared’ and 93 is called ‘nine cubed’. When no index is shown, the power is 1, i.e. 2 means 21 . Reciprocal The reciprocal of a number is when the index is −1 and its value is given by 1 divided by the base. Thus the reciprocal of 2 is 2−1 and its value is 12 or 0.5. Similarly, the reciprocal of 5 is 5−1 which means 15 or 0.2 Square root The square root of a number is when the index is 12 , and √ the square root of 2 is written as 21/2 or 2. The value of a square root is the value of the base which when √ multiplied by itself gives the number. √ Since 3 × 3 = 9, then 9 = 3. However, (−3) × (−3) = 9, so 9 = −3. There are always two answers when finding the square root of a number and this is shown by putting both a + and√a − sign in front of√ the answer to a square root problem. Thus 9 = ±3 and 41/2 = 4 = ±2, and so on.

(i) When multiplying two or more numbers having the same base, the indices are added. Thus 32 × 34 = 32+4 = 36 (ii) When a number is divided by a number having the same base, the indices are subtracted. Thus 35 = 35−2 = 33 32 (iii) When a number which is raised to a power is raised to a further power, the indices are multiplied. Thus (35 )2 = 35×2 = 310 (iv) When a number has an index of 0, its value is 1. Thus 30 = 1 (v) A number raised to a negative power is the reciprocal of 1 that number raised to a positive power. Thus 3−4 = 4 3 1 Similarly, −3 = 23 2 (vi) When a number is raised to a fractional power the denominator of the fraction is the root of the number and the numerator is the power. √ 3 Thus 82/3 = 82 = (2)2 = 4 √ √ 2 and 251/2 = 251 = 251 = ±5 (Note that

√

≡

√ 2

)

3.2 Worked problems on indices

Laws of indices When simplifying calculations involving indices, certain basic rules or laws can be applied, called the laws of indices. These are given below.

Problem 1. Evaluate: (a) 52 × 53 , (b) 32 × 34 × 3 and (c) 2 × 22 × 25

Indices and standard form

From law (i): (a) 52 × 53 = 5(2+3) = 55 = 5 × 5 × 5 × 5 × 5 = 3125

Problem 6.

Find the value of (a)

15

23 × 24 (32 )3 and (b) 5 7 2 ×2 3 × 39

(b) 32 × 34 × 3 = 3(2+4+1) = 37 = 3 × 3 × . . . to 7 terms

From the laws of indices:

= 2187

(a)

23 × 24 2(3+4) 27 1 1 = (7+5) = 12 = 27−12 = 2−5 = 5 = 7 5 2 ×2 2 2 2 32

(b)

(32 )3 32×3 36 1 1 = = = 36−10 = 3−4 = 4 = 3 × 39 31+9 310 3 81

(c) 2 × 22 × 25 = 2(1+2+5) = 28 = 256

Problem 2.

Find the value of: (a)

75 57 and (b) 4 3 7 5

From law (ii):

√ 4 = ±2 √ 4 (b) 163/4 = 163 = (2)3 = 8

57 = 5(7−4) = 53 = 125 54

Problem 3. Evaluate: (a) 52 × 53 ÷ 54 and (b) (3 × 35 ) ÷ (32 × 33 )

(Note that it does not matter whether the 4th root of 16 is found first or whether 16 cubed is found first–the same answer will result.) √ 3 (c) 272/3 = 272 = (3)2 = 9 (d) 9−1/2 =

From laws (i) and (ii): (a) 52 × 53 ÷ 54 = =

Evaluate (a) 41/2 (b) 163/4 (c) 272/3 (d) 9−1/2

(a) 41/2 =

75 (a) 3 = 7(5−3) = 72 = 49 7 (b)

Problem 7.

52 × 53 5(2+3) = 4 54 5 55 = 5(5−4) = 51 = 5 54

(b) (3 × 35 ) ÷ (32 × 33 ) = =

3 × 35 3(1+5) = 32 × 33 3(2+3) 36 = 36−5 = 31 = 3 35

Problem 4. Simplify: (a) (23 )4 (b) (32 )5 , expressing the answers in index form.

1 1 1 1 =± =√ = 91/2 3 9 ±3

Now try the following exercise Exercise 8

Further problems on indices (Answers on page 271)

In Problems 1 to 12, simplify the expressions given, expressing the answers in index form and with positive indices: 1. (a) 33 × 34

2. (a) 23 × 2 × 22 3. (a)

24 23

4. (a) 56 ÷ 53 5. (a) (72 )3

(a) (23 )4 = 23×4 = 212

6. (a) (153 )5

Problem 5.

Evaluate:

7. (a) (102 )3 104 × 102

From the laws of indices: (102 )3 10(2×3) 106 = = = 106−6 = 100 = 1 104 ×102 10(4+2) 106

8. (a) 9. (a) 10. (a)

(b) 72 × 74 × 7 × 73

37 32

(b)

From law (iii):

(b) (32 )5 = 32×5 = 310

(b) 42 × 43 × 44

(b) 713 /710 (b) (33 )2 (b) (172 )4

22 × 23 24

(b)

37 × 34 35

57 × 53

(b)

135 13 × 132

52

(9 × 32 )3 (3 × 27)2 5−2 5−4

(b)

(b)

(16 × 4)2 (2 × 8)3

32 × 3−4 33

16

Basic Engineering Mathematics

11. (a)

72 × 7−3 7 × 7−4

23 × 2−4 × 25 2 × 2−2 × 26

(b)

12. (a) 13 × 13−2 × 134 × 13−3

(b)

Problem 11. 5−7 × 52 5−8 × 53

32 × 5 5 33 × 5 3 + 2 2 3 3 3 ×5 +3 ×5 = 3 × 5 4 34 × 5 4 4 3 ×5 3 ×5 32 × 5 3 2

Evaluate

33 × 57 53 × 34

5

3

3

The laws of indices only apply to terms having the same base. Grouping terms having the same base, and then applying the laws of indices to each of the groups independently gives: 33 × 5 7 33 57 = × = 3(3−4) × 5(7−3) 53 × 3 4 34 53 = 3−1 × 54 =

Problem 9.

1 54 625 = 208 = 31 3 3

Find the value of

Problem 12.

23 × 35 × (72 )2 74 × 24 × 33

Problem 10.

41.5 = 43/2 =

Evaluate:

Hence

4 ×8 [(2) ] × (2 ) = 22 × 32−2/5 22 × (25 )−2/5 =2

3+1−2−(−2)

=

28 1 × 25 + 3 × 1 = 9×5 45

=

=

3 1/3

Since 7−3 =

=

2 ×2 22 × 2−2

= 2 = 16 4

32 × 55 34 × 54 + 33 × 53

3

3(2−2) × 5(5−3) 3(4−2) × 5(4−3) + 3(3−2) × 5(3−3) 32

30 × 52 × 51 + 31 × 50

25 25 = 45 + 3 48

7−3 × 34 , expressing the 3−2 × 75 × 5−2 answer in index form with positive indices. Problem 13.

Alternatively, 2 3/2

30 × 52 + 31 × 50 32 × 5 1

32 × 5 5 2 3 32 × 5 5 = 4 34 × 5 3 34 × 5 4 + 3 3 × 5 3 3 ×5 3 × 53 + 2 2 3 3 ×5 3 × 53

4 ×8 22 × 32−2/5

1 1 1 1 = 2/5 = √ = 2 = 5 2 32 2 4 32

1/3

=

=

41.5 × 81/3 8 × 2 16 = 16 = = 22 × 32−2/5 4 × 14 1

1.5

3(2−2) × 5(5−3) + 3(3−2) × 50 3(4−2) × 5(4−3)

Find the value of

1/3

√ √ 43 = 23 = 8, 81/3 = 3 8 = 2, 22 = 4 32−2/5

=

To simplify the arithmetic, each term is divided by the HCF of all the terms, i.e. 32 × 53 . Thus

23 × 35 × (72 )2 = 23−4 × 35−3 × 72×2−4 74 × 2 4 × 3 3 1 = 2−1 × 32 × 70 = × 32 × 1 2 9 1 = =4 2 2 1.5

32 × 55 + 33 × 53 34 × 54

Dividing each term by the HCF (i.e. highest common factor) of the three terms, i.e. 32 × 53 , gives:

3.3 Further worked problems on indices

Problem 8.

Evaluate:

1

Simplify

1 1 , = 32 73 3−2

and

1 = 52 5−2

7−3 × 34 34 × 3 2 × 52 = 5 −2 ×7 ×5 73 × 7 5

3−2

=

3(4+2) × 52 36 × 52 = 7(3+5) 78

then

Indices and standard form

162 × 9−2 Problem 14. Simplify expressing the 4 × 33 − 2−3 × 82 answer in index form with positive indices. Expressing the numbers in terms of their lowest prime numbers gives: (24 )2 × (32 )−2 162 × 9−2 = 2 3 −3 2 4×3 −2 ×8 2 × 33 − 2−3 × (23 )2 = =

28 × 3−4 22 × 33 − 2−3 × 26 28 × 3−4 × 33 − 23

22

Dividing each term by the HCF (i.e. 22 ) gives: 26 26 × 3−4 28 × 3−4 = = 22 × 3 3 − 2 3 33 − 2 34 (33 − 2) Problem 15.

Simplify

A fraction raised to a power means that both the numerator and the denominator of the fraction are raised to that power, 3 4 43 i.e. = 3 3 3 A fraction raised to a negative power has the same value as the inverse of the fraction raised to a positive power. −2 1 1 52 52 3 = 2 = 2 = 1 × 2 = 2 Thus, 5 3 3 3 3 2 5 5

Thus,

Exercise 9

Further problems on indices (Answers on page 271)

In Problems 1 and 2, simplify the expressions given, expressing the answers in index form and with positive indices: 33 × 52 7−2 × 3−2 1. (a) 4 (b) 5 4 5 ×3 3 × 74 × 7−3 8−2 × 52 × 3−4 252 × 24 × 9−2 −1 1 3. Evaluate (a) (b) 810.25 32 1/2 4 (c) 16(−1/4) (d) 9 2. (a)

42 × 93 83 × 34

(b)

In problems 4 to 10, evaluate the expressions given. 34

92 × 74 × 74 + 33 × 72

33 × 72 − 52 × 73 32 × 5 × 72 −2 3 2 1 − 2 3 8. 2 3 5

6.

giving the answer with positive indices.

Similarly,

Now try the following exercise

4.

3 −2 4 3 × 3 5 −3 2 5

−3 3 5 53 2 = = 3 5 2 2 3 −2 3 4 43 52 × × 2 3 3 5 = 3 33 −3 5 2 23 5 =

10.

5.

23

33 × 52 × 32 − 82 × 9

(24 )2 − 3−2 × 44 23 × 162 4 4 3 9. 2 2 9

7.

(32 )3/2 × (81/3 )2 (3)2 × (43 )1/2 × (9)−1/2

3.4 Standard form A number written with one digit to the left of the decimal point and multiplied by 10 raised to some power is said to be written in standard form. Thus: 5837 is written as 5.837 × 103 in standard form, and 0.0415 is written as 4.15 × 10−2 in standard form. When a number is written in standard form, the first factor is called the mantissa and the second factor is called the exponent. Thus the number 5.8 × 103 has a mantissa of 5.8 and an exponent of 103 (i) Numbers having the same exponent can be added or subtracted in standard form by adding or subtracting the mantissae and keeping the exponent the same. Thus:

43 52 23 × 2 × 3 3 3 5 3

(22 )3 × 23 29 = (3+2) = 5 (3−2) 3 ×5 3 ×5

17

2.3 × 104 + 3.7 × 104 = (2.3 + 3.7) × 104 = 6.0 × 104 and

5.9 × 10−2 − 4.6 × 10−2 = (5.9 − 4.6) × 10−2 = 1.3 × 10−2

18

Basic Engineering Mathematics

When the numbers have different exponents, one way of adding or subtracting the numbers is to express one of the numbers in non-standard form, so that both numbers have the same exponent. Thus: 2.3 × 104 + 3.7 × 103 = 2.3 × 104 + 0.37 × 104 = (2.3 + 0.37) × 104 = 2.67 × 10

(b) 5.491 × 104 = 5.491 × 10 000 = 54 910 (c) 9.84 × 100 = 9.84 × 1 = 9.84 (since 100 = 1) Problem 18. Express in standard form, correct to 3 significant figures: (a)

4

Alternatively, 2.3 × 104 + 3.7 × 103 = 23 000 + 3700 = 26 700

(a)

3 2 9 (b) 19 (c) 741 8 3 16

3 = 0.375, and expressing it in standard form gives: 8 0.375 = 3.75×10−1

= 2.67 × 104 (ii) The laws of indices are used when multiplying or dividing numbers given in standard form. For example, (2.5 × 103 ) × (5 × 102 ) = (2.5 × 5) × (103+2 ) = 12.5 × 105 or 1.25 × 106 Similarly,

6 × 104 6 × (104−2 ) = 4 × 102 = 1.5 × 102 1.5

2 (b) 19 = 19.6 = 1.97 × 10 in standard form, correct to 3 3 significant figures. 9 = 741.5625 = 7.42 × 102 in standard form, correct 16 to 3 significant figures.

(c) 741

Problem 19. Express the following numbers, given in standard form, as fractions or mixed numbers: (a) 2.5 × 10−1 (b) 6.25 × 10−2 (c) 1.354 × 102

3.5 Worked problems on standard form 25 1 2.5 = = 10 100 4 625 1 6.25 = = (b) 6.25 × 10−2 = 100 10 000 16 2 4 (c) 1.354 × 102 = 135.4 = 135 = 135 10 5 (a) 2.5 × 10−1 =

Problem 16.

Express in standard form:

(a) 38.71 (b) 3746 (c) 0.0124 For a number to be in standard form, it is expressed with only one digit to the left of the decimal point. Thus: (a) 38.71 must be divided by 10 to achieve one digit to the left of the decimal point and it must also be multiplied by 10 to maintain the equality, i.e. 38.71 = (b) 3746 =

38.71 × 10 = 3.871 × 10 in standard form 10 3746 × 1000 = 3.746 × 103 in standard form 1000

(c) 0.0124 = 0.0124 × form

100 1.24 = = 1.24 × 10−2 in standard 100 100

Problem 17. Express the following numbers, which are in standard form, as decimal numbers:

Now try the following exercise Exercise 10

Further problems on standard form (Answers on page 271)

In Problems 1 to 5, express in standard form: 1. (a) 73.9

(b) 28.4

2. (a) 2748

(b) 33 170

3. (a) 0.2401

(c) 197.72

(b) 0.0174

(c) 274 218 (c) 0.00923

4. (a) 1702.3 (b) 10.04 (c) 0.0109 7 3 1 1 (b) 11 (c) 130 (d) 5. (a) 2 8 5 32 In Problems 6 and 7, express the numbers given as integers or decimal fractions:

(a) 1.725 × 10−2 (b) 5.491 × 104 (c) 9.84 × 100

6. (a) 1.01 × 103 (d) 7 × 100

(b) 9.327 × 102

1.725 = 0.01725 100

7. (a) 3.89 × 10−2

(b) 6.741 × 10−1

(a) 1.725 × 10−2 =

(c) 5.41 × 104 (c) 8 × 10−3

Indices and standard form

3.6 Further worked problems on standard form Problem 20.

(a) (3.75 × 103 )(6 × 104 ) = (3.75 × 6)(103+4 ) = 22.50 × 107 = 2.25 × 108

Find the value of

(a) 7.9 × 10−2 − 5.4 × 10−2 (b) 8.3 × 103 + 5.415 × 103 and (c) 9.293 × 102 + 1.3 × 103 expressing the answers in standard form.

19

(b)

3.5 × 105 3.5 × 105−2 = 0.5 × 103 = 5 × 102 = 7 × 102 7

Now try the following exercise Exercise 11

Further problems on standard form (Answers on page 271)

Numbers having the same exponent can be added or subtracted by adding or subtracting the mantissae and keeping the exponent the same. Thus:

In Problems 1 to 4, find values of the expressions given, stating the answers in standard form:

(a) 7.9 × 10−2 − 5.4 × 10−2 = (7.9 − 5.4) × 10−2

1. (a) 3.7 × 102 + 9.81 × 102

= 2.5 × 10−2 (b) 8.3 × 103 + 5.415 × 103 = (8.3 + 5.415) × 103 = 13.715 × 103 = 1.3715 × 104

(b) 1.431 × 10−1 + 7.3 × 10−1 (c) 8.414 × 10−2 − 2.68 × 10−2 2. (a) 4.831 × 102 + 1.24 × 103 (b) 3.24 × 10−3 − 1.11 × 10−4 (c) 1.81 × 102 + 3.417 × 102 + 5.972 × 102

in standard form. (c) Since only numbers having the same exponents can be added by straight addition of the mantissae, the numbers are converted to this form before adding. Thus: 9.293 × 102 + 1.3 × 103 = 9.293 × 102 + 13 × 102

3. (a) (4.5 × 10−2 )(3 × 103 ) 4. (a)

6 × 10−3 3 × 10−5

(b)

(b) 2 × (5.5 × 104 )

(2.4 × 103 )(3 × 10−2 ) (4.8 × 104 )

5. Write the following statements in standard form.

= (9.293 + 13) × 102

(a) The density of aluminium is 2710 kg m−3

= 22.293 × 102

(b) Poisson’s ratio for gold is 0.44

= 2.2293×103

(c) The impedance of free space is 376.73 (d) The electron rest energy is 0.511 MeV

in standard form.

(e) Proton charge-mass ratio is 95 789 700 C kg−1

Alternatively, the numbers can be expressed as decimal fractions, giving:

(f) The normal volume 0.02241 m3 mol−1

of

a

perfect

gas

is

9.293 × 102 + 1.3 × 103 = 929.3 + 1300 = 2229.3 = 2.2293×103 in standard form as obtained previously. This method is often the ‘safest’ way of doing this type of problem.

Problem 21. (b)

Evaluate (a) (3.75 × 103 )(6 × 104 ) and

3.5 × 105 expressing answers in standard form. 7 × 102

3.7 Engineering notation and common prefixes Engineering notation is similar to scientific notation except that the power of ten is always a multiple of 3. For example, 0.00035 = 3.5 × 10−4 in scientific notation, but 0.00035 = 0.35 × 10−3 or 350 × 10−6 in engineering notation. Units used in engineering and science may be made larger or smaller by using prefixes that denote multiplication or division by a particular amount. The eight most common multiples, with

20

Basic Engineering Mathematics

Table 3.1 Prefix

Name

Meaning

T G M k m

tera giga mega kilo milli micro nano pico

multiply by 1 000 000 000 000 multiply by 1 000 000 000 multiply by 1 000 000 multiply by 1000 divide by 1000 divide by 1 000 000 divide by 1 000 000 000 divide by 1 000 000 000 000

µ

n p

(i.e. ×1012 ) (i.e. ×109 ) (i.e. ×106 ) (i.e. ×103 ) (i.e. ×10−3 ) (i.e. ×10−6 ) (i.e. ×10−9 ) (i.e. ×10−12 )

their meaning, are listed in Table 3.1, where it is noticed that the prefixes involve powers of ten which are all multiples of 3. For example, 5 MV

means 5 × 1 000 000 = 5 × 106 = 5 000 000 volts

3.6 k means 3.6 × 1000 = 3.6 × 103 = 3600 ohms 7.5 7.5 µC means 7.5 ÷ 1 000 000 = 6 10

or

7.5 × 10

−6

= 0.0000075 coulombs and 4 mA

means 4 × 10−3 or

4 4 = 0.004 amperes = 103 1000

0.00006 J = 0.06 mJ or 60 µJ 5 620 000 N = 5620 kN or 5.62 MN

and

−5

12 × 10

Now try the following exercise Exercise 12

Further problems on engineering notation and common prefixes (Answers on page 271)

1. Express the following in engineering notation and in prefix form: (a) 100 000 W (d) 225 × 10−4 V (f) 1.5 × 10−11 F

(b) 0.00054 A (c) 15 × 105 (e) 35 000 000 000 Hz (g) 0.000017 A (h) 46200 W

2. Rewrite the following as indicated: (a) 0.025 mA = ....... µA (c) 62 × 104 V = ....... kV

Similarly,

47 × 104 = 470 000 = 470 k

A calculator is needed for many engineering calculations, and having a calculator which has an ‘EXP’ and ‘ENG’ function is most helpful. For example, to calculate: 3 × 104 × 0.5 × 10−6 volts, input your calculator in the following order: (a) Enter ‘3’ (b) Press ‘EXP’ (c) Enter ‘4’ (d) Press‘ × ’(e) Enter ‘0.5’ (f) Press ‘EXP’ (g) Enter ‘−6’ (h) Press ‘=’ The answer is 0.015 V. Now press the ‘ENG’ button, and the answer changes to 15 × 10−3 V. The ‘ENG’ or ‘Engineering’ button ensures that the value is stated to a power of 10 that is a multiple of 3, enabling you, in this example, to express the answer as 15 mV.

or 0.47 M

A = 0.00012 A = 0.12 mA or 120 µA

(b) 1000 pF = ..... nF (d) 1 250 000 = .....M

3. Use a calculator to evaluate the following in engineering notation: (1.6 × 10−5 )(25 × 103 ) (a) 4.5 × 10−7 × 3 × 104 (b) (100 × 10−6 )

4 Calculations and evaluation of formulae

4.1 Errors and approximations (i) In all problems in which the measurement of distance, time, mass or other quantities occurs, an exact answer cannot be given; only an answer which is correct to a stated degree of accuracy can be given. To take account of this an error due to measurement is said to exist. (ii) To take account of measurement errors it is usual to limit answers so that the result given is not more than one significant figure greater than the least accurate number given in the data. (iii) Rounding-off errors can exist with decimal fractions. For example, to state that π = 3.142 is not strictly correct, but ‘π = 3.142 correct to 4 significant figures’ is a true statement. (Actually, π = 3.14159265 . . .) (iv) It is possible, through an incorrect procedure, to obtain the wrong answer to a calculation. This type of error is known as a blunder. (v) An order of magnitude error is said to exist if incorrect positioning of the decimal point occurs after a calculation has been completed. (vi) Blunders and order of magnitude errors can be reduced by determining approximate values of calculations. Answers which do not seem feasible must be checked and the calculation must be repeated as necessary. An engineer will often need to make a quick mental approximation for 49.1 × 18.4 × 122.1 a calculation. For example, may be 61.2 × 38.1 50 × 20 × 120 and then, by cancelling, approximated to 60 × 40 2 1 1 2 0 × 120 50 × = 50. An accurate answer somewhere 6 0 × 4 0 2 1 1 between 45 and 55 could therefore be expected. Certainly

an answer around 500 or 5 would not be expected. Actu49.1 × 18.4 × 122.1 ally, by calculator = 47.31, correct to 61.2 × 38.1 4 significant figures.

Problem 1. The area A of a triangle is given by A = 12 bh. The base b when measured is found to be 3.26 cm, and the perpendicular height h is 7.5 cm. Determine the area of the triangle.

Area of triangle = 12 bh = 12 × 3.26 × 7.5 = 12.225 cm2 (by calculator). The approximate value is 12 × 3 × 8 = 12 cm2 , so there are no obvious blunder or magnitude errors. However, it is not usual in a measurement type problem to state the answer to an accuracy greater than 1 significant figure more than the least accurate number in the data: this is 7.5 cm, so the result should not have more than 3 significant figures Thus area of triangle = 12.2 cm2

Problem 2. State which type of error has been made in the following statements: (a) 72 × 31.429 = 2262.9 (b) 16 × 0.08 × 7 = 89.6 (c) 11.714 × 0.0088 = 0.3247, correct to 4 decimal places. (d)

29.74 × 0.0512 = 0.12, correct to 2 significant figures. 11.89

22

Basic Engineering Mathematics

(a) 72 × 31.429 = 2262.888 (by calculator), hence a rounding-off error has occurred. The answer should have stated: 72 × 31.429 = 2262.9, correct to 5 significant figures or 2262.9, correct to 1 decimal place. 8 (b) 16 × 0.08 × 7 = 1 6× 25 × 7 100 4

Now try the following exercise Exercise 13

In Problems 1 to 5 state which type of error, or errors, have been made: 1. 25 × 0.06 × 1.4 = 0.21

24 32 × 7 224 = =8 = 25 25 25 = 8.96

2. 137 × 6.842 = 937.4 3.

Hence an order of magnitude error has occurred. (c) 11.714 × 0.0088 is approximately equal to 12 × 9 × 10−3 , i.e. about 108 × 10−3 or 0.108. Thus a blunder has been made. (d)

29.74 × 0.0512 30 × 5 × 10−2 150 ≈ = 11.89 12 12 × 102 =

1 15 = 120 8

or

Further problems on errors (Answers on page 271)

0.125

hence no order of magnitude error has occurred. 29.74 × 0.0512 However, = 0.128 correct to 3 signifi11.89 cant figures, which equals 0.13 correct to 2 significant figures.

24 × 0.008 = 10.42 12.6

4. For a gas pV = c. When pressure p = 1 03 400 Pa and V = 0.54 m3 then c = 55 836 Pa m3 . 5.

4.6 × 0.07 = 0.225 52.3 × 0.274

In Problems 6 to 8, evaluate the expressions approximately, without using a calculator. 6. 4.7 × 6.3 7.

2.87 × 4.07 6.12 × 0.96

8.

72.1 × 1.96 × 48.6 139.3 × 5.2

Hence a rounding-off error has occurred.

4.2 Use of calculator Problem 3. Without using a calculator, determine an approximate value of (a)

(a)

11.7 × 19.1 9.3 × 5.7

(b)

2.19 × 203.6 × 17.91 12.1 × 8.76

10 × 20 11.7 × 19.1 is approximately equal to , i.e. about 4 9.3 × 5.7 10 × 5 (By calculator, figures.)

11.7 × 19.1 = 4.22, correct to 3 significant 9.3 × 5.7

2.19 × 203.6 × 17.91 2 × 200 × 20 (b) ≈ = 2 × 2 × 20 after 12.1 × 8.76 10 × 10 cancelling i.e.

2.19 × 203.6 × 17.91 ≈ 80 12.1 × 8.76

2.19 × 203.6 × 17.91 (By calculator, = 75.3, correct to 12.1 × 8.76 3 significant figures.)

The most modern aid to calculations is the pocket-sized electronic calculator. With one of these, calculations can be quickly and accurately performed, correct to about 9 significant figures. The scientific type of calculator has made the use of tables and logarithms largely redundant. To help you to become competent at using your calculator check that you agree with the answers to the following problems: Problem 4. figures:

Evaluate the following, correct to 4 significant

(a) 4.7826 + 0.02713

(b) 17.6941 − 11.8762

(c) 21.93 × 0.012981 (a) 4.7826 + 0.02713 = 4.80973 = 4.810, correct to 4 significant figures. (b) 17.6941 − 11.8762 = 5.8179 = 5.818, correct to 4 significant figures. (c) 21.93 × 0.012981 = 0.2846733 . . . = 0.2847, 4 significant figures.

correct

to

Calculations and evaluation of formulae

Problem 5. places:

Evaluate the following, correct to 4 decimal

(a) 46.32 × 97.17 × 0.01258 (c)

(b)

Problem 8. places:

4.621 23.76

1 (62.49 × 0.0172) 2

(a) 46.32 × 97.17 × 0.01258 = 56.6215031 . . . = 56.6215, correct to 4 decimal places. (b)

(c)

4.621 = 0.19448653 . . . = 0.1945, correct to 4 decimal 23.76 places.

(a)

1 52.73

(c)

15 7.62 − 4.82

(b)

3.60 1.92

2 +

5.40 2.45

2

(2.37)2 = 106.785171 . . . = 106.785, correct to 3 decimal 0.0526 places.

(b)

3.60 2 5.40 2 + = 8.37360084 . . . = 8.374, correct 1.92 2.45 to 3 decimal places.

(c)

15 = 0.43202764 . . . = 0.432, correct to 3 decimal 7.62 − 4.82 places.

Evaluate the following, correct to 3 decimal

(b)

1 0.0275

(c)

Problem 9. figures:

1 1 + 4.92 1.97

(a) (a)

(2.37)2 0.0526

1 (62.49 × 0.0172) = 0.537414 = 0.5374, correct to 4 2 decimal places.

Problem 6. places:

Evaluate the following, correct to 3 decimal

(a)

(a)

23

1 = 0.01896453 . . . = 0.019, correct to 3 decimal 52.73 places.

(b)

1 = 36.3636363 . . . = 36.364, correct to 3 decimal 0.0275 places.

(c)

1 1 + = 0.71086624 . . . = 0.711, 4.92 1.97 decimal places.

correct

to

3

Problem 7. Evaluate the following, expressing the answers in standard form, correct to 4 significant figures. (a) (0.00451)2

(a) (0.00451)2 = 2.03401 × 10−5 = 2.034 × 10−5 , correct to 4 significant figures. (b) 631.7 − (6.21 + 2.95)2 = 547.7944 = 5.477944 × 102 = 5.478 × 102 , correct to 4 significant figures. to

√

54.62

(c)

√

546.2

√ 5.462 = 2.3370922 . . . = 2.337, correct to 4 significant figures. √ (b) 54.62 = 7.39053448 . . . = 7.391, correct to 4 significant figures. √ (c) 546.2 = 23.370922 . . . = 23.37, correct to 4 significant figures.

(c)

(c) 46.272 − 31.792

(b)

(a)

(a)

correct

5.462

Problem 10. places:

(b) 631.7 − (6.21 + 2.95)2

(c) 46.272 − 31.792 = 1130.3088 = 1.130 × 103 , 4 significant figures.

√

Evaluate the following, correct to 4 significant

√

Evaluate the following, correct to 3 decimal

0.007328

(b)

√

52.91 −

√

31.76

(1.6291 × 104 )

√ 0.007328 = 0.08560373 = 0.086, correct to 3 decimal places. √ √ (b) 52.91 − 31.76 = 1.63832491 . . . = 1.638, correct to 3 decimal places. (c) (1.6291 × 104 ) = (16291) = 127.636201 . . . = 127.636, correct to 3 decimal places. (a)

24

Basic Engineering Mathematics

Problem 11. cant figures: (a) 4.723

Evaluate the following, correct to 4 signifi-

(b) (0.8316)4

(c)

(76.212 − 29.102 )

(a) 4.723 = 105.15404 . . . = 105.2, correct to 4 significant figures. (b) (0.8316)4 = 0.47825324 . . . = 0.4783, correct to 4 significant figures. (c) (76.212 − 29.102 ) = 70.4354605 . . . = 70.44, correct to 4 significant figures. Problem 12. cant figures: (a) (c)

Evaluate the following, correct to 3 signifi-

6.092 √ 25.2 × 7

(b)

√ 3

47.291

Now try the following exercise Exercise 14

In Problems 1 to 3, use a calculator to evaluate the quantities shown correct to 4 significant figures: 1. (a) 3.2492 (b) 73.782 (c) 311.42 (d) 0.06392 √ √ √ 2. (a) 4.735 (b) 35.46 (c) 73280 √ (d) 0.0256 1 1 1 1 (c) (d) (b) 3. (a) 48.46 0.0816 1.118 7.768 In Problems 4 to 11, use a calculator to evaluate correct to 4 significant figures: 4. (a) 43.27 × 12.91

6.092 √ = 0.74583457 . . . = 0.746, correct to 25.2 × 7 3 significant figures. √ (b) 3 47.291 = 3.61625876 . . . = 3.62, correct to 3 significant figures. (c) (7.2132 + 6.4183 + 3.2914 ) = 20.8252991 . . . = 20.8, correct to 3 significant figures. (a)

137.6 552.9

(b)

11.82 × 1.736 0.041

7. (a)

1 17.31

(b)

1 0.0346

(a) (5.176 × 10−3 )2 1.974 × 101 × 8.61 × 10−2 3.462 (c) (1.792 × 10−4 )

4

(b) 3.4764 (c) 0.1245 √ √ √ 9. (a) 347.1 (b) 7632 (c) 0.027 √ (d) 0.004168

11. (a)

1.974 × 101 × 8.61 × 10−2 3.462

(a)

4 = 0.05808887 . . .

= 5.8089 × 10−2 , correct to 4 decimal places (c) (1.792 × 10−4 ) = 0.0133865 . . . = 1.3387 × 10−2 , correct to 4 decimal places.

24.68 × 0.0532 7.412

14.323 21.682

(b)

3 (b)

17.332

0.2681 × 41.22 32.6 × 11.89

4

4.8213 − 15.86 × 11.6

12. Evaluate correct to 3 decimal places:

(b)

(a) (5.176 × 10−3 )2 = 2.679097 . . . × 10−5 = 2.6791 × 10−5 , correct to 4 decimal places.

1 147.9

(c)

8. (a) 13.63

10. (a) Problem 13. Evaluate the following, expressing the answers in standard form, correct to 4 decimal places:

(b) 15.76 ÷ 4.329

6. (a)

(b)

(b) 54.31 × 0.5724

5. (a) 127.8 × 0.0431 × 19.8

(7.2132 + 6.4183 + 3.2914 )

Further problems on use of calculator (Answers on page 271)

29.12 (5.81)2 − (2.96)2

(b)

√

53.98 −

√

21.78

13. Evaluate correct to 4 significant figures: (15.62)2 (a) √ 29.21 × 10.52 (b)

(6.9212 + 4.8163 − 2.1614 )

14. Evaluate the following, expressing the answers in standard form, correct to 3 decimal places: (a) (8.291 × 10−2 )2 (b) (7.623 × 10−3 )

Calculations and evaluation of formulae

4.3 Conversion tables and charts It is often necessary to make calculations from various conversion tables and charts. Examples include currency exchange rates, imperial to metric unit conversions, train or bus timetables, production schedules and so on.

Problem 14. Currency exchange rates for five countries are shown in Table 4.1 Table 4.1 France Japan Norway Switzerland U.S.A.

£1 = 1.50 euros £1 = 175 yen £1 = 11.25 kronor £1 = 2.20 francs £1 = 1.82 dollars ($)

£27.80 = 27.80 × 1.50 euros = 41.70 euros (b) £1 = 175 yen, hence £23 = 23 × 175 yen = 4025 yen (c) £1 = 11.25 kronor, hence 6412.5 = £570 6412.5 kronor = £ 11.25 (d) £1 = 1.82 dollars, hence £90 = 90 × 1.82 dollars = $163.80 (e) £1 = 2.20 Swiss francs, hence 2794 = £1270 2794 franc = £ 2.20 Problem 15. Some approximate imperial to metric conversions are shown in Table 4.2 Table 4.2

capacity

(a) the number of millimetres in 9.5 inches, (b) a speed of 50 miles per hour in kilometres per hour, (c) the number of miles in 300 km, (d) the number of kilograms in 30 pounds weight, (e) the number of pounds and ounces in 42 kilograms (correct to the nearest ounce), (f ) the number of litres in 15 gallons, and

(a) 9.5 inches = 9.5 × 2.54 cm = 24.13 cm 24.13 cm = 24.13 × 10 mm = 241.3 mm (b) 50 m.p.h. = 50 × 1.61 km/h = 80.5 km/h (c) 300 km = (d) 30 lb =

300 miles = 186.3 miles 1.61

30 kg = 13.64 kg 2.2

(e) 42 kg = 42 × 2.2 lb = 92.4 lb 0.4 lb = 0.4 × 16 oz = 6.4 oz = 6 oz, correct to the nearest ounce. Thus 42 kg = 92 lb 6 oz, correct to the nearest ounce. (f ) 15 gallons = 15 × 8 pints = 120 pints 120 pints =

(a) £1 = 1.50 euros, hence

weight

Use the table to determine:

(g) the number of gallons in 40 litres.

Calculate: (a) how many French euros £27.80 will buy, (b) the number of Japanese yen which can be bought for £23, (c) the pounds sterling which can be exchanged for 6412.5 Norwegian kronor, (d) the number ofAmerican dollars which can be purchased for £90, and (e) the pounds sterling which can be exchanged for 2794 Swiss francs.

length

25

1 inch = 2.54 cm 1 mile = 1.61 km 2.2 lb = 1 kg (1 lb = 16 oz) 1.76 pints = 1 litre (8 pints = 1 gallon)

120 litres = 68.18 litres 1.76

(g) 40 litres = 40 × 1.76 pints = 70.4 pints 70.4 pints =

70.4 gallons = 8.8 gallons 8

Now try the following exercise Exercise 15

Further problems conversion tables and charts (Answers on page 272)

1. Currency exchange rates listed in a newspaper included the following: Italy Japan Australia Canada Sweden

£1 = 1.52 euro £1 = 180 yen £1 = 2.40 dollars £1 = $2.35 £1 = 13.90 kronor

Calculate (a) how many Italian euros £32.50 will buy, (b) the number of Canadian dollars that can be purchased for £74.80, (c) the pounds sterling which can be exchanged for 14 040 yen, (d) the pounds sterling which can be exchanged for 1751.4 Swedish kronor, and (e) the Australian dollars which can be bought for £55

26

Basic Engineering Mathematics

Table 4.3 Liverpool, Hunt’s Cross and Warrington → Manchester

Reproduced with permission of British Rail

2. Below is a list of some metric to imperial conversions. Length

2.54 cm = 1 inch 1.61 km = 1 mile

Weight

1 kg = 2.2 lb (1 lb = 16 ounces)

Capacity

1 litre = 1.76 pints (8 pints = 1 gallon)

Use the list to determine (a) the number of millimetres in 15 inches, (b) a speed of 35 mph in km/h, (c) the

number of kilometres in 235 miles, (d) the number of pounds and ounces in 24 kg (correct to the nearest ounce), (e) the number of kilograms in 15 lb, (f ) the number of litres in 12 gallons and (g) the number of gallons in 25 litres. 3. Deduce the following information from the BR train timetable shown in Table 4.3: (a) At what time should a man catch a train at Mossley Hill to enable him to be in Manchester Piccadilly by 8.15 a.m.?

Calculations and evaluation of formulae

(b) A girl leaves Hunts Cross at 8.17 a.m. and travels to Manchester Oxford Road. How long does the journey take. What is the average speed of the journey? (c) A man living at Edge Hill has to be at work at Trafford Park by 8.45 a.m. It takes him 10 minutes to walk to his work from Trafford Park station. What time train should he catch from Edge Hill?

4.4 Evaluation of formulae The statement v = u + at is said to be a formula for v in terms of u, a and t. v, u, a and t are called symbols. The single term on the left-hand side of the equation, v, is called the subject of the formulae. Provided values are given for all the symbols in a formula except one, the remaining symbol can be made the subject of the formula and may be evaluated by using a calculator.

Problem 19. The area, A, of a circle is given by A = πr 2 . Determine the area correct to 2 decimal places, given radius r = 5.23 m.

A = πr 2 = π(5.23)2 = π(27.3529) Hence area, A = 85.93 m2 , correct to 2 decimal places.

Problem 20. The power P watts dissipated in an electrical V2 . Evalucircuit may be expressed by the formula P = R ate the power, correct to 3 significant figures, given that V = 17.48 V and R = 36.12 .

P=

(17.48)2 305.5504 V2 = = R 36.12 36.12

Hence power, figures. Problem 16. In an electrical circuit the voltage V is given by Ohm’s law, i.e. V = IR. Find, correct to 4 significant figures, the voltage when I = 5.36 A and R = 14.76 .

27

P = 8.46W,

correct

to

3

significant

Problem 21. The volume V cm3 of a right circular cone is given by V = 13 πr 2 h. Given that r = 4.321 cm and h = 18.35 cm, find the volume, correct to 4 significant figures.

V = IR = (5.36)(14.76) Hence voltage V = 79.11V, correct to 4 significant figures.

Problem 17. The surface area A of a hollow cone is given by A = π rl. Determine, correct to 1 decimal place, the surface area when r = 3.0 cm and l = 8.5 cm. A = π rl = π(3.0)(8.5) cm2 Hence surface area A = 80.1 cm2 , correct to 1 decimal place. Problem 18. Velocity v is given by v = u + at. If u = 9.86 m/s, a = 4.25 m/s2 and t = 6.84 s, find v, correct to 3 significant figures.

v = u + at = 9.86 + (4.25)(6.84) = 9.86 + 29.07 = 38.93 Hence velocity υ = 38.9 m/s, correct to 3 significant figures.

V = 13 πr 2 h = 13 π(4.321)2 (18.35) = 13 π(18.671041)(18.35) Hence volume,V = 358.8 cm3 , correct to 4 significant figures.

Problem 22. Force F newtons is given by the formula Gm1 m2 , where m1 and m2 are masses, d their distance F= d2 apart and G is a constant. Find the value of the force given that G = 6.67 × 10−11 , m1 = 7.36, m2 = 15.5 and d = 22.6. Express the answer in standard form, correct to 3 significant figures.

F=

Gm1 m2 (6.67 × 10−11 )(7.36)(15.5) = 2 d (22.6)2 =

1.490 (6.67)(7.36)(15.5) = (1011 )(510.76) 1011

Hence force F = 1.49 × 10−11 newtons, correct to 3 significant figures.

28

Basic Engineering Mathematics

Problem 23. The time ofswing t seconds, of a simple penl dulum is given by t = 2π . Determine the time, correct g to 3 decimal places, given that l = 12.0 and g = 9.81. t = 2π

l = (2)π g

12.0 9.81

√ = (2)π 1.22324159 = (2)π (1.106002527) Hence time t = 6.950 seconds, correct to 3 decimal places.

Problem 24. Resistance, R, varies with temperature according to the formula R = R0 (1 + αt). Evaluate R, correct to 3 significant figures, given R0 = 14.59, α = 0.0043 and t = 80.

R = R0 (1 + αt) = 14.59[1 + (0.0043)(80)] = 14.59(1 + 0.344) = 14.59(1.344)

8. Find the area A of a triangle, given A = 12 bh, when the base length b is 23.42 m and the height h is 53.7 m. 9. Resistance R2 is given by R2 = R1 (1 + αt). Find R2 , correct to 4 significant figures, when R1 = 220, α = 0.00027 and t = 75.6. mass . Find the density when the mass is 10. Density = volume 2.462 kg and the volume is 173 cm3 . Give the answer in units of kg/m3 . 11. Velocity = frequency × wavelength. Find the velocity when the frequency is 1825 Hz and the wavelength is 0.154 m. 12. Evaluate resistance RT , given 1 1 1 1 = + + RT R1 R2 R3 when R1 = 5.5 , R2 = 7.42 and R3 = 12.6 . 13. Find the total cost of 37 calculators costing £12.65 each and 19 drawing sets costing £6.38 each. force × distance . Find the power when a force time of 3760 N raises an object a distance of 4.73 m in 35 s.

14. Power =

Hence resistance, R = 19.6 , correct to 3 significant figures.

15. The potential difference, V volts, available at battery terminals is given by V = E − Ir. Evaluate V when E = 5.62, I = 0.70 and R = 4.30.

Now try the following exercise

16. Given force F = 12 m(v 2 − u2 ), find F when m = 18.3, v = 12.7 and u = 8.24.

Exercise 16 Further problems on evaluation of formulae (Answers on page 272) 1. The area A of a rectangle is given by the formula A = lb. Evaluate the area when l = 12.4 cm and b = 5.37 cm. 2. The circumference C of a circle is given by the formula C = 2πr. Determine the circumference given π = 3.14 and r = 8.40 mm. 3. A formula used in connection with gases is R = (PV )/T . Evaluate R when P = 1500, V = 5 and T = 200. 4. The velocity of a body is given by v = u + at. The initial velocity u is measured when time t is 15 seconds and found to be 12 m/s. If the acceleration a is 9.81 m/s2 calculate the final velocity v. 5. Calculate the current I in an electrical circuit, when I = V /R amperes when the voltage V is measured and found to be 7.2 V and the resistance R is 17.7 . 6. Find the distance s, given that s = 12 gt 2 . Time t = 0.032 seconds and acceleration due to gravity g = 9.81 m/s2 . 7. The energy stored in a capacitor is given by E = 12 CV 2 joules. Determine the energy when capacitance C = 5 × 10−6 farads and voltage V = 240 V.

17. The current I amperes flowing in a number of cells is nE . Evaluate the current when n = 36, given by I = R + nr E = 2.20, R = 2.80 and r = 0.50. 18. The time, t seconds, of oscillation for a simple pendul . Determine the time when lum is given by t = 2π g π = 3.142, l = 54.32 and g = 9.81. 19. Energy, E joules, is given by the formula E = 12 LI 2 . Evaluate the energy when L = 5.5 and I = 1.2. 20. The current I amperes in an a.c. circuit is given by V I= . Evaluate the current when V = 250, 2 (R + X 2 ) R = 11.0 and X = 16.2. 21. Distance s metres is given by the formula s = ut + 12 at 2 . If u = 9.50, t = 4.60 and a = −2.50, evaluate the distance. 22. The

area, A, of any triangle is given by a+b+c . A = [s(s − a)(s − b)(s − c)] where s = 2 Evaluate the area given a = 3.60 cm, b = 4.00 cm and c = 5.20 cm.

Calculations and evaluation of formulae

23. Given that a = 0.290, b = 14.86, c = 0.042, d = 31.8 and e = 0.650, evaluate v given that v=

ab d − c e

(a) 5.9 × 102 + 7.31 × 102

(b) 2.75 × 10−2 − 2.65 × 10−3 (6) 73 × 0.009 = 4.18 × 10−2 correct? If 4. Is the statement 15.7 not, what should it be? (2)

Assignment 2 This assignment covers the material contained in Chapters 3 and 4. The marks for each question are shown in brackets at the end of each question. 1. Evaluate the following: (a)

23 × 2 × 22 24

(d) (27)−1/3

(b)

(23 × 16)2 (8 × 2)3

(c)

−2 3 2 − 2 9 (e) 2 2 3

1 42

−1

(b) 0.076

(c)

145 25

5. Evaluate the following, each correct to 4 significant figures: √ 1 (c) 0.0527 (6) (a) 61.222 (b) 0.0419 6. Evaluate the following, each correct to 2 decimal places: 3 36.22 × 0.561 (a) 27.8 × 12.83 14.692 (b) (7) √ 17.42 × 37.98 7. If 1.6 km = 1 mile, determine the speed of 45 miles/hour in kilometres per hour. (3)

(14)

8. The area A of a circle is given by A = πr 2 . Find the area of a circle of radius r = 3.73 cm, correct to 2 decimal places. (3) 9. Evaluate B, correct to 3 significant figures, when W = 7.20, v = 10.0 and g = 9.81, given that

2. Express the following in standard form: (a) 1623

3. Determine the value of the following, giving the answer in standard form:

(6)

B=

W v2 2g

(3)

29

5 Computer numbering systems

5.1 Binary numbers The system of numbers in everyday use is the denary or decimal system of numbers, using the digits 0 to 9. It has ten different digits (0, 1, 2, 3, 4, 5, 6, 7, 8 and 9) and is said to have a radix or base of 10. The binary system of numbers has a radix of 2 and uses only the digits 0 and 1.

Problem 2.

Convert 0.10112 to a decimal fraction.

0.10112 = 1 × 2−1 + 0 × 2−2 + 1 × 2−3 + 1 × 2−4 1 1 1 1 +0× 2 +1× 3 +1× 4 2 2 2 2 1 1 1 = + + 2 8 16 = 0.5 + 0.125 + 0.0625 =1×

5.2 Conversion of binary to denary

= 0.687510

The denary number 234.5 is equivalent to 2 × 102 + 3 × 101 + 4 × 100 + 5 × 10−1 i.e. is the sum of terms comprising: (a digit) multiplied by (the base raised to some power). In the binary system of numbers, the base is 2, so 1101.1 is equivalent to:

Problem 3.

Convert 101.01012 to a denary number.

101.01012 = 1 × 22 + 0 × 21 + 1 × 20 + 0 × 2−1 + 1 × 2−2 + 0 × 2−3 + 1 × 2−4 = 4 + 0 + 1 + 0 + 0.25 + 0 + 0.0625

1 × 23 + 1 × 22 + 0 × 21 + 1 × 20 + 1 × 2−1

= 5.312510

Thus the denary number equivalent to the binary number 1101.1 is 1 8 + 4 + 0 + 1 + , that is 13.5 2 i.e. 1101.12 = 13.510 , the suffixes 2 and 10 denoting binary and denary systems of numbers respectively. Problem 1.

Now try the following exercise Exercise 17

Convert 110112 to a denary number.

Further problems on conversion of binary to denary numbers (Answers on page 272)

From above: 110112 = 1 × 24 + 1 × 23 + 0 × 22 + 1 × 21 + 1 × 20 = 16 + 8 + 0 + 2 + 1 = 2710

In Problems 1 to 4, convert the binary numbers given to denary numbers. 1. (a) 110 2. (a) 10101

(b) 1011

(c) 1110

(b) 11001

(d) 1001

(c) 101101

(d) 110011

Computer numbering systems

3. (a) 0.1101

(b) 0.11001

4. (a) 11010.11

(c) 0.00111

(d) 0.01011

(b) 10111.011

(c) 110101.0111

(d) 11010101.10111

5.3 Conversion of denary to binary An integer denary number can be converted to a corresponding binary number by repeatedly dividing by 2 and noting the remainder at each stage, as shown below for 3910

From above, repeatedly dividing by 2 and noting the remainder gives:

2 47

Remainder

2 23

1

2 11

1

2 5

1

2 2

1

2 1 0

0 1 1

2 39

Remainder

2 19

1

2 9

1

2

4

1

2

2

0

2

1

0

0

1

0

1

1

1

1

Thus 4710 = 1011112 Problem 5.

Convert 0.4062510 to a binary number.

From above, repeatedly multiplying by 2 gives:

(most significant bit)→ 1 0 0 1 1 1 ←(least significant bit)

The result is obtained by writing the top digit of the remainder as the least significant bit, (a bit is a binary digit and the least significant bit is the one on the right). The bottom bit of the remainder is the most significant bit, i.e. the bit on the left.

0.40625 ⫻ 2 ⫽

0. 8125

0.8125 ⫻ 2 ⫽

1. 625

0.625

⫻ 2 ⫽

1. 25

Thus 3910 = 1001112

0.25

⫻ 2 ⫽

0. 5

The fractional part of a denary number can be converted to a binary number by repeatedly multiplying by 2, as shown below for the fraction 0.625

0.5

× 2 ⫽

1. 0

0.625 ⫻ 2 ⫽

1.

250

0.250 ⫻ 2 ⫽

0.

500

0.500 ⫻ 2 ⫽

1.

000

(most significant bit). 1

0

1 (least significant bit)

0

1

1

0

1

i.e. 0.4062510 = 0.011012 Problem 6.

Convert 58.312510 to a binary number.

The integer part is repeatedly divided by 2, giving:

2 58

Remainder

2 29

0

For fractions, the most significant bit of the result is the top bit obtained from the integer part of multiplication by 2. The least significant bit of the result is the bottom bit obtained from the integer part of multiplication by 2.

2 14

1

2

7

0

2

3

1

Thus 0.62510 = 0.1012

2

1

1

0

1

Problem 4.

31

Convert 4710 to a binary number.

1

1

1

0

1

0

32

Basic Engineering Mathematics

The fractional part is repeatedly multiplied by 2 giving:

0.3125 ⫻ 2 0.625 ⫻ 2 0.25 ⫻ 2 0.5 ⫻ 2

⫽ ⫽ ⫽ ⫽

0.625 1.25 0.5 1.0

Thus 49310 = 7558 The fractional part of a denary number can be converted to an octal number by repeatedly multiplying by 8, as shown below for the fraction 0.437510

0.4375 ⫻ 8 ⫽

3. 5

⫻8 ⫽

4. 0

0.5

. 0 1 0 1

.3 4 Thus 58.312510 = 111010.01012 For fractions, the most significant bit is the top integer obtained by multiplication of the denary fraction by 8, thus Now try the following exercise

0.437510 = 0.348

Exercise 18 Further problems on conversion of denary to binary numbers (Answers on page 272)

The natural binary code for digits 0 to 7 is shown in Table 5.1, and an octal number can be converted to a binary number by writing down the three bits corresponding to the octal digit.

In Problems 1 to 4, convert the denary numbers given to binary numbers. 1. (a) 5

(b) 15

2. (a) 31

(b) 42

3. (a) 0.25

(c) 19 (c) 57

(b) 0.21875

4. (a) 47.40625 (c) 53.90625

Table 5.1

(d) 29

Octal digit

Natural binary number

0 1 2 3 4 5 6 7

000 001 010 011 100 101 110 111

(d) 63 (c) 0.28125

(d) 0.59375

(b) 30.8125 (d) 61.65625

5.4 Conversion of denary to binary via octal For denary integers containing several digits, repeatedly dividing by 2 can be a lengthy process. In this case, it is usually easier to convert a denary number to a binary number via the octal system of numbers. This system has a radix of 8, using the digits 0, 1, 2, 3, 4, 5, 6 and 7. The denary number equivalent to the octal number 43178 is i.e.

4 × 83 + 3 × 82 + 1 × 81 + 7 × 80 4 × 512 + 3 × 64 + 1 × 8 + 7 × 1

or

Thus 4378 = 100 011 1112 and 26.358 = 010 110.011 1012 The ‘0’ on the extreme left does not signify anything, thus 26.358 = 10 110.011 1012 Conversion of denary to binary via octal is demonstrated in the following worked problems.

225510 Problem 7.

Convert 371410 to a binary number, via octal.

An integer denary number can be converted to a corresponding octal number by repeatedly dividing by 8 and noting the remainder at each stage, as shown below for 49310

Dividing repeatedly by 8, and noting the remainder gives:

8 493

8 3714

8 8

61

Remainder 5

7

5

0

7 7

5

5

Remainder

8

464

2

8

58

0

8

7

2

0

7 7

2

0

2

Computer numbering systems

72028 = 111 010 000 0102

From Table 5.1,

371410 = 111 010 000 0102

i.e. Problem 8. via octal.

Convert 0.5937510 to a binary number,

Multiplying repeatedly by 8, and noting the integer values, gives:

0.59375 ⫻ 8 ⫽ 0.75 ⫻ 8 ⫽

4.75 6.00

33

Problem 10. Convert 11 110 011.100 012 to a denary number via octal. Grouping the binary number in three’s from the binary point gives: 011 110 011.100 0102 Using Table 5.1 to convert this binary number to an octal number gives: 363.428 and 363.428 = 3 × 82 + 6 × 81 + 3 × 80 + 4 × 8−1 + 2 × 8−2 = 192 + 48 + 3 + 0.5 + 0.03125 = 243.5312510

. 4 6 Now try the following exercise 0.5937510 = 0.468

Thus

0.468 = 0.100 1102

From Table 5.1,

Exercise 19

0.5937510 = 0.100 112

i.e. Problem 9. via octal.

Convert 5613.9062510 to a binary number,

Further problems on conversion between denary and binary numbers via octal (Answers on page 272)

In Problems 1 to 3, convert the denary numbers given to binary numbers, via octal. 1. (a) 343

(b) 572

(c) 1265

The integer part is repeatedly divided by 8, noting the remainder, giving:

2. (a) 0.46875

8

5613

8

701

5

4. Convert the following binary numbers to denary numbers via octal:

8

87

5

5. (a) 111.011 1

8

10

7

8

1

2

0

1

Remainder

2

7

5

5

This octal number is converted to a binary number, (see Table 5.1) 127558 = 001 010 111 101 1012 561310 = 1 010 111 101 1012

The fractional part is repeatedly multiplied by 8, and noting the integer part, giving:

0.90625 ⫻ 8 ⫽ 0.25 ⫻ 8 ⫽

7.25 2.00 .7 2

This octal fraction is converted to a binary number, (see Table 5.1)

(b) 514.4375

(c) 1716.78125

(b) 101 001.01

5.5 Hexadecimal numbers The complexity of computers requires higher order numbering systems such as octal (base 8) and hexadecimal (base 16) which are merely extensions of the binary system. A hexadecimal numbering system has a radix of 16 and uses the following 16 distinct digits: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E and F ‘A’ corresponds to 10 in the denary system, B to 11, C to 12, and so on. To convert from hexadecimal to decimal: For example

0.9062510 = 0.111 012

Thus, 5613.9062510 =1 010 111 101 101.111 012

1A16 = 1 × 161 + A × 160 = 1 × 161 + 10 × 1 = 16 + 10 = 26

0.728 = 0.111 0102 i.e.

(c) 0.71875

(c) 1 110 011 011 010.001 1

1

i.e.

3. (a) 247.09375

(b) 0.6875

i.e.

1A16 = 2610

Similarly,

2E16 = 2 × 161 + E × 160 = 2 × 161 + 14 × 160 = 32 + 14 = 4610

34

and

Basic Engineering Mathematics

1BF16 = 1 × 162 + B × 161 + F × 160

(b) 3F16 = 3 × 161 + F × 160 = 3 × 16 + 15 × 1

= 1 × 162 + 11 × 161 + 15 × 160

= 48 + 15 = 63 Thus 3F16 = 6310

= 256 + 176 + 15 = 44710 Table 5.2 compares decimal, binary, octal and hexadecimal numbers and shows, for example, that 2310 = 101112 = 278 = 1716

Problem 12. Convert the following hexadecimal numbers into their decimal equivalents: (a) C916 (b) BD16 (a) C916 = C × 161 + 9 × 160 = 12 × 16 + 9 × 1

Table 5.2

= 192 + 9 = 201

Decimal

Binary

Octal

Hexadecimal

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

0000 0001 0010 0011 0100 0101 0110 0111 1000 1001 1010 1011 1100 1101 1110 1111 10000 10001 10010 10011 10100 10101 10110 10111 11000 11001 11010 11011 11100 11101 11110 11111 100000

0 1 2 3 4 5 6 7 10 11 12 13 14 15 16 17 20 21 22 23 24 25 26 27 30 31 32 33 34 35 36 37 40

0 1 2 3 4 5 6 7 8 9 A B C D E F 10 11 12 13 14 15 16 17 18 19 1A 1B 1C 1D 1E 1F 20

Thus C916 = 20110 (b) BD16 = B × 161 + D × 160 = 11 × 16 + 13 × 1 = 176 + 13 = 189 Thus BD16 = 18910 Problem 13.

Convert 1A4E16 into a denary number.

1A4E16 = 1 × 163 + A × 162 + 4 × 161 + E × 160 =1 × 163 + 10 × 162 + 4 × 161 + 14 × 160 = 1 × 4096 + 10 × 256 + 4 × 16 + 14 × 1 = 4096 + 2560 + 64 + 14 = 6734 Thus 1A4E16 = 673410

To convert from decimal to hexadecimal: This is achieved by repeatedly dividing by 16 and noting the remainder at each stage, as shown below for 2610

16 26

Remainder

1

10 ⬅ A16

0

1 ⬅ 116

16

most significant bit → 1 A ← least significant bit Hence 2610 = 1A16 Similarly, for 44710

16 447 Problem 11. Convert the following hexadecimal numbers into their decimal equivalents: (a) 7A16 (b) 3F16

Remainder

16

27

15 ⬅ F16

16

1 0

11 ⬅ B16 1 ⬅ 116 1 B F

(a) 7A16 = 7 × 161 + A × 160 = 7 × 16 + 10 × 1 = 112 + 10 = 122 Thus 7A16 = 12210

Thus 44710 = 1BF16

Computer numbering systems

Problem 14. Convert the following decimal numbers into their hexadecimal equivalents: (a) 3710 (b) 10810

35

To convert from hexadecimal to binary: The above procedure is reversed, thus, for example, 6CF316 = 0110 1100 1111 0011from Table 5.2

(a) 16 37

16

2 0

i.e.

Remainder 5 ⫽ 516 2 ⫽ 216

Problem 16. Convert the following binary numbers into their hexadecimal equivalents:

most significant bit → 2 5 ← least significant bit

(a) 110101102

Hence 3710 = 2516 (b) 16 108

16

Remainder

6

12 ⫽ C16

0

6 ⫽ 616

6CF316 = 1101100111100112

(b) 11001112

(a) Grouping bits in fours from the right gives: and assigning hexadecimal symbols to each group gives:

6 C

0101

0110

D 6 from Table 5.2

Thus, 110101102 = D616

Hence 10810 = 6C16

Problem 15. Convert the following decimal numbers into their hexadecimal equivalents: (a) 16210 (b) 23910

(b) Grouping bits in fours from the right gives: and assigning hexadecimal symbols to each group gives:

0110

0111

6 7 from Table 5.2

Thus, 11001112 = 6716 (a) 16 162

16

10 0

Remainder 2 ⫽ 216 10 ⫽ A16

Problem 17. Convert the following binary numbers into their hexadecimal equivalents:

A 2

(a) 110011112

Hence 16210 = A216 (b) 16 239

16

14 0

(b) 1100111102

(a) Grouping bits in fours from the right gives: and assigning hexadecimal symbols to each group gives:

Remainder 15 ⫽ F16 14 ⫽ E16

1100

1111

C F from Table 5.2

Thus, 110011112 = CF16

E F (b) Grouping bits in fours from the right gives: 0001 1001 1110 and assigning hexadecimal symbols to each group gives: 1 9 E from Table 5.2 Thus, 1100111102 = 19E16

Hence 23910 = EF16

To convert from binary to hexadecimal: The binary bits are arranged in groups of four, starting from right to left, and a hexadecimal symbol is assigned to each group. For example, the binary number 1110011110101001 is initially grouped in fours as:

Problem 18. Convert the following hexadecimal numbers into their binary equivalents: (a) 3F16 (b) A616

1110 0111 1010 1001

and a hexadecimal symbol assigned to each group Hence 11100111101010012 = E7A916

E

7

A

9

from Table 5.2

(a) Spacing out hexadecimal digits gives: and converting each into binary gives: Thus, 3F16 = 1111112

3 F 0011 1111 from Table 5.2

36

Basic Engineering Mathematics

(b) Spacing out hexadecimal digits gives: and converting each into binary gives:

A 6 1010 0110 from Table 5.2

Thus, A616 = 101001102

Problem 19. Convert the following hexadecimal numbers into their binary equivalents: (a) 7B16 (b) 17D16

(a) Spacing out hexadecimal digits gives: and converting each into binary gives:

7 B 0111 1011 from Table 5.2

Thus, 7B16 = 11110112 (b) Spacing out hexadecimal digits gives: and converting each into binary gives:

1

7

D

0001 0111 1101 from Table 5.2 Thus, 17D16 = 1011111012

Now try the following exercise Exercise 20

Further problems on hexadecimal numbers (Answers on page 272)

In Problems 1 to 4, convert the given hexadecimal numbers into their decimal equivalents. 1. E716

2. 2C16

3. 9816

4. 2F116

In Problems 5 to 8, convert the given decimal numbers into their hexadecimal equivalents. 5. 5410

6. 20010

7. 9110

8. 23810

In Problems 9 to 12, convert the given binary numbers into their hexadecimal equivalents. 9. 110101112

10. 111010102

11. 100010112

12. 101001012

In Problems 13 to 16, convert the given hexadecimal numbers into their binary equivalents. 13. 3716

14. ED16

15. 9F16

16. A2116

6 Algebra

6.1 Basic operations

Problem 3.

Algebra is that part of mathematics in which the relations and properties of numbers are investigated by means of general symbols. For example, the area of a rectangle is found by multiplying the length by the breadth; this is expressed algebraically as A = l × b, where A represents the area, l the length and b the breadth. The basic laws introduced in arithmetic are generalized in algebra. Let a, b, c and d represent any four numbers. Then: a + (b + c) = (a + b) + c a(bc) = (ab)c a+b=b+a ab = ba a(b + c) = ab + ac a+b a b (vi) = + c b c (vii) (a + b)(c + d) = ac + ad + bc + bd (i) (ii) (iii) (iv) (v)

Problem 1. and c = 5

Find the sum of 3x, 2x, −x and −7x

The sum of the positive terms is: 3x + 2x = 5x The sum of the negative terms is: x + 7x = 8x Taking the sum of the negative terms from the sum of the positive terms gives: 5x − 8x = −3x Alternatively 3x + 2x + (−x) + (−7x) = 3x + 2x − x − 7x = −3x Problem 4.

Find the sum of 4a, 3b, c, −2a, −5b and 6c

Each symbol must be dealt with individually.

Evaluate 3ab − 2bc + abc when a = 1, b = 3

Replacing a, b and c with their numerical values gives: 3ab − 2bc + abc = 3 × 1 × 3 − 2 × 3 × 5 + 1 × 3 × 5

For the ‘a’ terms:

+4a − 2a = 2a

For the ‘b’ terms:

+3b − 5b = −2b

For the ‘c’ terms:

+c + 6c = 7c

Thus 4a + 3b + c + (−2a) + (−5b) + 6c = 4a + 3b + c − 2a − 5b + 6c = 2a − 2b + 7c Problem 5. Find the sum of 5a − 2b, 2a + c, 4b − 5d and b − a + 3d − 4c

= 9 − 30 + 15 = −6 Problem 2. Find the value of 4p2 qr 3 , given that p = 2, q = 12 and r = 1 12 Replacing p, q and r with their numerical values gives: 3 4p2 qr 3 = 4(2)2 12 32 = 4×2×2×

1 2

×

3 2

×

3 2

×

3 2

= 27

The algebraic expressions may be tabulated as shown below, forming columns for the a’s, b’s, c’s and d’s. Thus: +5a − 2b +2a + c + 4b − 5d − a + b − 4c + 3d Adding gives:

6a + 3b − 3c − 2d

38

Basic Engineering Mathematics

Problem 6.

Subtract 2x + 3y −4z from x − 2y + 5z

2x2 + x − 3 is called the dividend and x − 1 the divisor. The usual layout is shown below with the dividend and divisor both arranged in descending powers of the symbols.

x − 2y + 5z 2x + 3y − 4z

x−1

−x− 5y + 9z

Subtracting gives:

2x + 3 2x2 + x − 3 2x2 − 2x 3x − 3 3x − 3

(Note that +5z − −4z = +5z + 4z = 9z) An alternative method of subtracting algebraic expressions is to ‘change the signs of the bottom line and add’. Hence: x − 2y + 5z −2x − 3y + 4z −x − 5y + 9z

Adding gives:

Problem 7.

Multiply 2a + 3b by a + b

Each term in the first expression is multiplied by a, then each term in the first expression is multiplied by b, and the two results are added. The usual layout is shown below.

0

0

Dividing the first term of the dividend by the first term of the 2x2 gives 2x, which is put above the first term of divisor, i.e. x the dividend as shown. The divisor is then multiplied by 2x, i.e. 2x(x − 1) = 2x2 − 2x, which is placed under the dividend as shown. Subtracting gives 3x − 3. The process is then repeated, i.e. the first term of the divisor, x, is divided into 3x, giving 3, which is placed above the dividend as shown. Then 3(x − 1) = 3x − 3, which is placed under the 3x − 3. The remainder, on subtraction, is zero, which completes the process.

2a + 3b a+ b

Thus (2x2 + x − 3) ÷ (x − 1) = (2x + 3)

Multiplying by a → Multiplying by b →

2a2+ 3ab + 2ab + 3b2

(A check can be made on this answer by multiplying (2x + 3) by (x − 1), which equals (2x2 + x − 3)

Adding gives:

2a2+ 5ab + 3b2 Problem 11.

Problem 8.

Simplify

x3 + y3 x+y

Multiply 3x − 2y2 + 4xy by 2x − 5y 3x − 2y 2x − 5y

Multiplying by 2x → Multiplying by −5y →

(1) (4) (7) x2 − xy + y2

x + y x3 + 0 + 0 + y3 x3 + x2 y

+ 4xy

2

6x2 − 4xy2 + 8x2 y − 20xy2

6x − 24xy + 8x y −15xy + 10y 2

Adding gives:

2

− x2 y

−15xy + 10y3 2

+ y3

− x y − xy 2

2

3

xy2 + y3 xy2 + y3

Problem 9.

Simplify 2p ÷ 8pq

2p ÷ 8pq means

2p . This can be reduced by cancelling as in 8pq

arithmetic. 2p 2×p 1 Thus: = = 8pq 8 × p × p 4q Problem 10.

Divide 2x2 + x − 3 by x − 1

0

Thus

0

(1) x into x3 goes x2 . Put x2 above x3 (2) x2 (x + y) = x3 + x2 y (3) Subtract (4) x into −x2 y goes −xy. Put −xy above dividend (5) −xy(x + y) = −x2 y − xy2 (6) Subtract (7) x into xy2 goes y2 . Put y2 above dividend (8) y2 (x + y) = xy2 + y3 (9) Subtract

x 3 + y3 = x 2 − xy + y2 x+y

The zeros shown in the dividend are not normally shown, but are included to clarify the subtraction process and to keep similar terms in their respective columns. Problem 12.

Divide 4a3 − 6a2 b + 5b3 by 2a − b

Algebra

2 2

2a − 2ab − b 2a − b 4a3 − 6a2 b 4a3 − 2a2 b

+ 5b

3

− 4a2 b + 5b3 − 4a2 b + 2ab2 − 2ab2 + 5b3 − 2ab2 + b3 4b

(iii) (am )n = amn √ (iv) am/n = n am 1 (v) a−n = n a (vi) a0 = 1 Problem 13.

4a3 − 6a2 b + 5b3 = 2a2 − 2ab − b2 , remainder 4b3 . 2a − b Alternatively, the answer may be expressed as 4b3 2a − b

Now try the following exercise

Grouping like terms gives: a3 × a × b2 × b3 × c × c5 Using the first law of indices gives: a3+1 × b2+3 × c1+5 i.e. a4 × b5 × c6 = a4 b5 c6 Problem 14.

a1/2 b2 c−2 × a(1/6) b(1/2) c = a(1/2)+(1/6) × b2+(1/2) × c−2+1 = a2/3 b5/2 c−1

1. Find the value of 2xy + 3yz − xyz, when x = 2, y = −2 and z = 4 2 2. Evaluate 3pq2 r 3 when p = , q = −2 and r = −1 3 3. Find the sum of 3a, −2a, −6a, 5a and 4a 1 4 4. Simplify c + 2c − c − c 3 6 1 1 5. Find the sum of 3x, 2y, −5x, 2z, − y, − x 2 4 6. Add together 2a + 3b + 4c, −5a − 2b + c, 4a − 5b − 6c

and

7. Add together 3d + 4e, −2e + f , 2d − 3f , 4d − e + 2f − 3e

Thus

8. From 4x − 3y + 2z subtract x + 2y − 3z b b 3 9. Subtract a − + c from − 4a − 3c 2 3 2 10. Multiply 3x + 2y by x − y 11. Multiply 2a − 5b + c by 3a + b 12. Simplify (i) 3a ÷ 9ab (ii) 4a2 b ÷ 2a

Simplify a1/2 b2 c−2 × a1/6 b1/2 c

Using the first law of indices,

Exercise 21 Further problems on basic operations (Answers on page 272)

13. Divide 2x2 + xy − y2 by x + y

Simplify a3 b2 c × ab3 c5

3

Thus

2a2 − 2ab − b2 +

a 3 b2 c 4 Problem 15. Simplify and evaluate when a = 3, abc−2 1 b = and c = 2 8 Using the second law of indices, a3 = a3−1 = a2 , a

b2 = b2−1 = b b

c4 = c4−−2 = c6 c−2 a3 b2 c4 = a2 bc6 abc−2

When a = 3, b = 18 and c = 2, a2 bc6 = (3)2 18 (2)6 = (9) 18 (64) = 72 p1/2 q2 r 2/3 and evaluate when p1/4 q1/2 r 1/6 p = 16, q = 9 and r = 4, taking positive roots only. Problem 16.

Simplify

14. Divide 3p2 − pq − 2q2 by p − q Using the second law of indices gives:

6.2 Laws of Indices

p(1/2)−(1/4) q2−(1/2) r (2/3)−(1/6) = p1/4 q3/2 r 1/2 When p = 16, q = 9 and r = 4,

The laws of indices are: (i) am × an = am+n am (ii) n = am−n a

39

p1/4 q3/2 r 1/2 = (16)1/4 (9)3/2 (4)1/2 √ √ √ 4 = ( 16)( 93 )( 4) = (2)(33 )(2) = 108

40

Basic Engineering Mathematics

Problem 17. Simplify

x2 y3 + xy2 xy

Algebraic expressions of the form Thus

a b a+b can be split into + . c c c

x2 y3 + xy2 x2 y3 xy2 = + = x2−1 y3−1 + x1−1 y2−1 xy xy xy = xy2 + y (since x0 = 1, from the sixth law of indices.)

Using the second law of indices gives: m3 n6 = m3−2 n6−1 = mn5 m 2 n1 √ √ √ √ 3 Problem 22. Simplify (a3 b c5 )( a b2 c3 ) and evaluate when a = 14 , b = 6 and c = 1 Using the fourth law of indices, the expression can be written as: (a3 b1/2 c5/2 )(a1/2 b2/3 c3 ) Using the first law of indices gives:

2

Problem 18.

Simplify

x y xy2 − xy

The highest common factor (HCF) of each of the three terms comprising the numerator and denominator is xy. Dividing each term by xy gives: x2 y xy

x2 y x = = 2 − xy y−1 xy xy − xy xy

xy2

Problem 19.

Simplify

ab2

a2 b − a1/2 b3

The HCF of each of the three terms is a1/2 b. Dividing each term by a1/2 b gives: a2 b a3/2 a2 b a1/2 b = = ab2 − a1/2 b3 a1/2 b3 ab2 a1/2 b − b2 − 1/2 1/2 a b a b Problem 20.

a3+(1/2) b(1/2)+(2/3) c(5/2)+3 = a7/2 b7/6 c11/2 It is usual to express the answer in the same form as the question. Hence √ √ 6 a7/2 b7/6 c11/2 = a7 b7 c11 When a = 14 , b = 64 and c = 1, √ √ √ √ 1 7 √ 6 6 a7 b7 c11 = ( 647 )( 111 ) 4 7 = 12 (2)7 (1) = 1 Problem 23. Simplify (a3 b)(a−4 b−2 ), expressing the answer with positive indices only. Using the first law of indices gives: a3+−4 b1+−2 = a−1 b−1 1 1 Using the fifth law of indices gives: a−1 b−1 = +1 +1 = a b ab d 2 e2 f 1/2 expressing the answer (d 3/2 ef 5/2 )2 with positive indices only. Problem 24.

Simplify

Simplify ( p3 )1/2 (q2 )4 Using the third law of indices gives:

Using the third law of indices gives: p3×(1/2) q2×4 = p(3/2) q8

d 2 e2 f 1/2 d 2 e2 f 1/2 = (d 3/2 ef 5/2 )2 d 3 e2 f 5 Using the second law of indices gives:

2 3

Problem 21.

Simplify

(mn ) (m1/2 n1/4 )4

The brackets indicate that each letter in the bracket must be raised to the power outside.

d 2−3 e2−2 f (1/2)−5 = d −1 e0 f −9/2 = d −1 f (−9/2) since e0 = 1 from the sixth law of indices 1 = 9/2 from the fifth law of indices df

Using the third law of indices gives: (mn2 )3 m1×3 n2×3 m 3 n6 = (1/2)×4 (1/4)×4 = 2 1 1/2 1/4 4 (m n ) m m n n

Problem 25.

√ (x2 y1/2 )( x 3 y2 ) Simplify (x5 y3 )1/2

Algebra

Using the third and fourth laws of indices gives: √ (x2 y1/2 )(x1/2 y2/3 ) (x2 y1/2 )( x 3 y2 ) = 3 1/2 5 (x y ) x5/2 y3/2 Using the first and second laws of indices gives: x2+(1/2)−(5/2) y(1/2)+(2/3)−(3/2) = x0 y−1/3 1 1 = y−1/3 or or √ 3 y y1/3 from the fifth and sixth laws of indices.

Now try the following exercise Exercise 22 Further problems on laws of indices (Answers on page 272) 1 1. Simplify (x2 y3 z)(x3 yz 2 ) and evaluate when x = , y = 2 2 and z = 3 2. Simplify (a3/2 bc−3 )(a1/2 b−1/2 c) and evaluate when a = 3, b = 4 and c = 2 1 a5 bc3 3 3. Simplify 2 3 2 and evaluate when a = , b = and a b c 2 2 2 c= 3 In Problems 4 to 11, simplify the given expressions: a2 b + a3 b a2 b2

4.

x1/5 y1/2 z 1/3 −1/2 x y1/3 z −1/6

5.

6.

p3 q 2 2 pq − p2 q

7. (a2 )1/2 (b2 )3 (c1/2 )3

8.

(abc)2 (a2 b−1 c−3 )3

√ √ √ √ 3 9. ( x y3 z 2 )( x y3 z 3 )

10. (e2 f 3 )(e−3 f −5 ), expressing the answer with positive indices only 11.

(a3 b1/2 c−1/2 )(ab)1/3 √ √ ( a3 b c)

Problem 26. Remove the brackets and simplify the expression (3a + b) + 2(b + c) − 4(c + d) Both b and c in the second bracket have to be multiplied by 2, and c and d in the third bracket by −4 when the brackets are removed. Thus: (3a + b) + 2(b + c) − 4(c + d) = 3a + b + 2b + 2c − 4c − 4d Collecting similar terms together gives: 3a + 3b − 2c − 4d Problem 27.

Simplify a2 − (2a − ab) − a(3b + a)

When the brackets are removed, both 2a and −ab in the first bracket must be multiplied by −1 and both 3b and a in the second bracket by −a. Thus a2 − (2a − ab) − a(3b + a) = a2 − 2a + ab − 3ab − a2 Collecting similar terms together gives: −2a − 2ab. Since −2a is a common factor, the answer can be expressed as −2a(1 + b) Problem 28.

Simplify (a + b)(a − b)

Each term in the second bracket has to be multiplied by each term in the first bracket. Thus: (a + b)(a − b) = a(a − b) + b(a − b) = a2 − ab + ab − b2 = a2 − b2 Alternatively

a + b a − b

Multiplying by a → a2 + ab Multiplying by −b → − ab − b2 Adding gives:

a2

− b2

6.3 Brackets and factorization When two or more terms in an algebraic expression contain a common factor, then this factor can be shown outside of a bracket. For example ab + ac = a(b + c)

Problem 29. Remove the brackets from the expression (x − 2y)(3x + y2 )

(x − 2y)(3x + y2 ) = x(3x + y2 ) − 2y(3x + y2 ) = 3x 2 + xy2 − 6xy − 2y3

which is simply the reverse of law (v) of algebra on page 36, and 6px + 2py − 4pz = 2p(3x + y − 2z) Problem 30. This process is called factorization.

41

Simplify (2x − 3y)2

42

Basic Engineering Mathematics

(2x − 3y)2 = (2x − 3y)(2x − 3y) = 2x(2x − 3y) − 3y(2x − 3y) = 4x2 − 6xy − 6xy + 9y2 = 4x 2 − 12xy + 9y2 2x − 3y 2x − 3y

Alternatively,

Multiplying by 2x → 4x2 − 6xy Multiplying by −3y → − 6xy + 9y2 4x2 − 12xy + 9y2

Adding gives:

Factorizing gives: −6x(x + y) since −6x is common to both terms. Problem 34. Factorize (a) xy − 3xz (b) 4a2 + 16ab3 (c) 3a2 b − 6ab2 + 15ab For each part of this problem, the HCF of the terms will become one of the factors. Thus: (a) xy − 3xz = x( y − 3z) (b) 4a2 + 16ab3 = 4a(a + 4b3 )

Problem 31.

Remove the brackets from the expression

(c) 3a2 b − 6ab2 + 15ab = 3ab(a − 2b + 5)

2[ p − 3(q + r) + q ] 2

2

In this problem there are two brackets and the ‘inner’ one is removed first. Hence 2[ p2 − 3(q + r) + q2 ] = 2[ p2 − 3q − 3r + q2 ] = 2p2 − 6q − 6r + 2q2

Problem 35.

Factorize ax − ay + bx − by

The first two terms have a common factor of a and the last two terms a common factor of b. Thus: ax − ay + bx − by = a(x − y) + b(x − y)

Problem 32. Remove the brackets and simplify the expression: 2a − [3{2(4a − b) − 5(a + 2b)} + 4a] Removing the innermost brackets gives:

The two newly formed terms have a common factor of (x − y). Thus: a(x − y) + b(x − y) = (x−y)(a+ b)

2a − [3{8a − 2b − 5a − 10b} + 4a] Collecting together similar terms gives: 2a − [3{3a − 12b} + 4a]

2a − [9a − 36b + 4a]

2ax − 3ay + 2bx − 3by = a(2x − 3y) + b(2x − 3y)

Collecting together similar terms gives: 2a − [13a − 36b]

2a − 13a + 36b 36b − 11a

(2x − 3y) is now a common factor thus: a(2x − 3y) + b(2x − 3y) = (2x−3y)(a+b)

Removing the outer brackets gives:

or

Factorize 2ax − 3ay + 2bx − 3by

a is a common factor of the first two terms and b a common factor of the last two terms. Thus:

Removing the ‘curly’ brackets gives:

i.e. −11a + 36b

Problem 36.

(see law (iii), page 37)

Alternatively, 2x is a common factor of the original first and third terms and −3y is a common factor of the second and fourth terms. Thus: 2ax − 3ay + 2bx − 3by = 2x(a + b) − 3y(a + b)

Problem 33.

Simplify x(2x − 4y) − 2x(4x + y)

Removing brackets gives: 2x2 − 4xy − 8x2 − 2xy

(a + b) is now a common factor thus: 2x(a + b) − 3y(a + b) = (a + b)(2x − 3y) as before.

Collecting together similar terms gives: −6x2 − 6xy

Problem 37.

Factorize x3 + 3x2 − x − 3

Algebra

x2 is a common factor of the first two terms, thus: x3 + 3x2 − x − 3 = x2 (x + 3) − x − 3

Multiplication is performed before addition and subtraction thus: 2a + 5a × 3a − a = 2a + 15a2 − a = a + 15a2 = a(1 + 15a)

−1 is a common factor of the last two terms, thus: x2 (x + 3) − x − 3 = x2 (x + 3) − 1(x + 3) (x + 3) is now a common factor, thus: x (x + 3) − 1(x + 3) = (x + 3)(x − 1) 2

2

43

Problem 39.

Simplify (a + 5a) × 2a − 3a

The order of precedence is brackets, multiplication, then subtraction. Hence (a + 5a) × 2a − 3a = 6a × 2a − 3a = 12a2 − 3a = 3a(4a − 1)

Now try the following exercise Exercise 23 Further problems on brackets and factorization (Answers on page 273) In Problems 1 to 13, remove the brackets and simplify where possible: 1. (x + 2y) + (2x − y)

Problem 40.

Simplify a + 5a × (2a − 3a)

The order of precedence is brackets, multiplication, then subtraction. Hence a + 5a × (2a − 3a) = a + 5a × −a = a + −5a2

2. (4a + 3y) − (a − 2y)

= a − 5a2 = a(1 − 5a)

3. 2(x − y) − 3(y − x) 4. 2x2 − 3(x − xy) − x(2y − x) 5. 2( p + 3q − r) − 4(r − q + 2p) + p

Problem 41.

Simplify a ÷ 5a + 2a − 3a

6. (a + b)(a + 2b) 7. ( p + q)(3p − 2q) 8. (i) (x − 2y)2

(ii) (3a − b)2

9. 3a(b + c) + 4c(a − b) 10. 2x + [ y − (2x + y)] 11. 3a + 2[a − (3a − 2)] 12. 2 − 5[a(a − 2b) − (a − b)2 ] 13. 24p − [2(3(5p − q) − 2( p + 2q)) + 3q] In Problems 14 to 17, factorize: 14. (i) pb + 2pc

(ii) 2q2 + 8qn

15. (i) 21a2 b2 − 28ab (ii) 2xy2 + 6x2 y + 8x3 y 16. (i) ay + by + a + b (ii) px + qx + py + qy 17. (i) ax − ay + bx − by

(ii) 2ax + 3ay − 4bx − 6by

The order of precedence is division, then addition and subtraction. Hence a a ÷ 5a + 2a − 3a = + 2a − 3a 5a 1 1 = + 2a − 3a = − a 5 5 Problem 42.

Simplify a ÷ (5a + 2a) − 3a

The order of precedence is brackets, division and subtraction. Hence a ÷ (5a + 2a) − 3a = a ÷ 7a − 3a a 1 = − 3a = − 3a 7a 7 Problem 43.

Simplify a ÷ (5a + 2a − 3a)

6.4 Fundamental laws and precedence The order of precedence is brackets, then division. Hence: The laws of precedence which apply to arithmetic also apply to algebraic expressions. The order is Brackets, Of, Division, Multiplication, Addition and Subtraction (i.e. BODMAS) Problem 38.

Simplify 2a + 5a × 3a − a

a ÷ (5a + 2a − 3a) = a ÷ 4a =

Problem 44.

1 a = 4a 4

Simplify 3c + 2c × 4c + c ÷ 5c − 8c

44

Basic Engineering Mathematics

The order of precedence is division, multiplication, addition and subtraction. Hence: 3c + 2c × 4c + c ÷ 5c − 8c = 3c + 2c × 4c +

c − 8c 5c

1 − 8c 5 1 or = 8c2 − 5c + 5 1 c(8c − 5) + 5

= 3c + 8c2 +

Problem 45.

Simplify (3c + 2c) 4c + c ÷ 5c − 8c

The order of precedence is brackets, division, multiplication, addition and subtraction. Hence

The order of precedence is brackets, division and multiplication. Hence (3c + 2c)(4c + c) ÷ (5c − 8c) = 5c × 5c ÷ −3c 5c = 5c × −3c 5 25 = 5c × − = − c 3 3

Problem 48.

Simplify (2a − 3) ÷ 4a + 5 × 6 − 3a

The bracket around the (2a − 3) shows that both 2a and −3 have to be divided by 4a, and to remove the bracket the expression is written in fraction form. Hence: (2a − 3) ÷ 4a + 5 × 6 − 3a =

(3c + 2c)4c + c ÷ 5c − 8c = 5c × 4c + c ÷ 5c − 8c = 5c × 4c +

=

c − 8c 5c

=

1 = 20c + − 8c or 5 1 4c(5c − 2) + 5 2

Problem 46.

= =

Simplify 3c + 2c × 4c + c ÷ (5c − 8c) Problem 49.

The order of precedence is brackets, division, multiplication and addition. Hence: 3c + 2c × 4c + c ÷ (5c − 8c) =3c + 2c × 4c + c ÷ −3c c =3c + 2c × 4c + −3c 1 c = Now −3c −3

Simplify

1 3

2a − 3 + 5 × 6 − 3a 4a 2a − 3 + 30 − 3a 4a 2a 3 − + 30 − 3a 4a 4a 1 3 − + 30 − 3a 2 4a 1 3 30 − − 3a 2 4a

of 3p + 4p(3p − p)

Applying BODMAS, the expression becomes 1 3

of 3p + 4p × 2p

and changing ‘of’ to ‘×’ gives: 1 3

× 3p + 4p × 2p

i.e. p + 8p2

p(1 + 8p)

or

Multiplying numerator and denominator by −1 gives 1 × −1 −3 × −1

Now try the following exercise i.e. −

1 3

Hence: 3c + 2c × 4c +

Problem 47.

1 c = 3c + 2c × 4c − −3c 3 1 = 3c + 8c2 − or 3 1 c(3 + 8c) − 3

Simplify (3c + 2c)(4c + c) ÷ (5c − 8c)

Exercise 24

Further problems on fundamental laws and precedence (Answers on page 273)

In Problems 1 to 12, simplify: 1. 2x ÷ 4x + 6x

2. 2x ÷ (4x + 6x)

3. 3a − 2a × 4a + a

4. (3a − 2a)4a + a

5. 3a − 2a(4a + a)

6. 2y + 4 ÷ 6y + 3 × 4 − 5y

7. (2y + 4) ÷ 6y + 3 × 4 − 5y 8. 2y + 4 ÷ 6y + 3(4 − 5y) 9. 3 ÷ y + 2 ÷ y + 1

Algebra

10. p2 − 3pq × 2p ÷ 6q + pq 11. (x + 1)(x − 4) ÷ (2x + 2) 12.

1 4

of 2y + 3y(2y − y)

45

Problem 51. Hooke’s law states that stress σ is directly proportional to strain ε within the elastic limit of a material. When, for mild steel, the stress is 25 × 106 pascals, the strain is 0.000125. Determine (a) the coefficient of proportionality and (b) the value of strain when the stress is 18 × 106 pascals.

6.5 Direct and inverse proportionality An expression such as y = 3x contains two variables. For every value of x there is a corresponding value of y. The variable x is called the independent variable and y is called the dependent variable. When an increase or decrease in an independent variable leads to an increase or decrease of the same proportion in the dependent variable this is termed direct proportion. If y = 3x then y is directly proportional to x, which may be written as y ∝ x or y = kx, where k is called the coefficient of proportionality (in this case, k being equal to 3). When an increase in an independent variable leads to a decrease of the same proportion in the dependent variable (or vice versa) this is termed inverse proportion. If y is inversely 1 proportional to x then y ∝ or y = k/x. Alternatively, k = xy, x that is, for inverse proportionality the product of the variables is constant. Examples of laws involving direct and inverse proportional in science include: (i) Hooke’s law, which states that within the elastic limit of a material, the strain produced is directly proportional to the stress, σ , producing it, i.e. ε ∝ σ or ε = kσ . (ii) Charles’s law, which states that for a given mass of gas at constant pressure the volume V is directly proportional to its thermodynamic temperature T , i.e. V ∝ T or V = kT . (iii) Ohm’s law, which states that the current I flowing through a fixed resistor is directly proportional to the applied voltage V , i.e. I ∝ V or I = kV . (iv) Boyle’s law, which states that for a gas at constant temperature, the volume V of a fixed mass of gas is inversely proportional to its absolute pressure p, i.e. p ∝ (1/V ) or p = k/V , i.e. pV = k Problem 50. If y is directly proportional to x and y = 2.48 when x = 0.4, determine (a) the coefficient of proportionality and (b) the value of y when x = 0.65 (a) y ∝ x, i.e. y = kx. If y = 2.48 when x = 0.4, 2.48 = k(0.4) Hence the coefficient of proportionality, k=

2.48 = 6.2 0.4

(b) y = kx, hence, when x = 0.65, y = (6.2)(0.65) = 4.03

(a) σ ∝ ε, i.e. σ = kε, from which k = σ/ε. Hence the coefficient of proportionality, k=

25 × 106 = 200 × 109 pascals 0.000125

(The coefficient of proportionality k in this case is called Young’s Modulus of Elasticity) (b) Since σ = kε, ε = σ/k Hence when σ = 18 × 106 , strain ε=

18 × 106 = 0.00009 200 × 109

Problem 52. The electrical resistance R of a piece of wire is inversely proportional to the cross-sectional area A. When A = 5 mm2 , R = 7.02 ohms. Determine (a) the coefficient of proportionality and (b) the cross-sectional area when the resistance is 4 ohms.

1 , i.e. R = k/A or k = RA. Hence, when R = 7.2 and A A = 5, the coefficient of proportionality, k = (7.2)(5) = 36

(a) R ∝

(b) Since k = RA then A = k/R When R = 4, the cross sectional area, A =

36 = 9 mm2 4

Problem 53. Boyle’s law states that at constant temperature, the volume V of a fixed mass of gas is inversely proportional to its absolute pressure p. If a gas occupies a volume of 0.08 m3 at a pressure of 1.5 × 106 pascals determine (a) the coefficient of proportionality and (b) the volume if the pressure is changed to 4 × 106 pascals..

1 (a) V ∝ , i.e. V = k/p or k = pV p Hence the coefficient of proportionality, k = (1.5 × 106 )(0.08) = 0.12 × 106 (b) Volume V =

k 0.12 × 106 = = 0.03 m3 p 4 × 106

46

Basic Engineering Mathematics

Now try the following exercise

Assignment 3 Exercise 25 Further problems on direct and inverse proportionality (Answers on page 273) 1. If p is directly proportional to q and p = 37.5 when q = 2.5, determine (a) the constant of proportionality and (b) the value of p when q is 5.2. 2. Charles’s law states that for a given mass of gas at constant pressure the volume is directly proportional to its thermodynamic temperature. A gas occupies a volume of 2.25 litres at 300 K. Determine (a) the constant of proportionality, (b) the volume at 420 K and (c) the temperature when the volume is 2.625 litres. 3. Ohm’s law states that the current flowing in a fixed resistor is directly proportional to the applied voltage. When 30 volts is applied across a resistor the current flowing through the resistor is 2.4 × 10−3 amperes. Determine (a) the constant of proportionality, (b) the current when the voltage is 52 volts and (c) the voltage when the current is 3.6 × 10−3 amperes. 4. If y is inversely proportional to x and y = 15.3 when x = 0.6, determine (a) the coefficient of proportionality, (b) the value of y when x is 1.5, and (c) the value of x when y is 27.2. 5. Boyle’s law states that for a gas at constant temperature, the volume of a fixed mass of gas is inversely proportional to its absolute pressure. If a gas occupies a volume of 1.5 m3 at a pressure of 200 × 103 pascals, determine (a) the constant of proportionality, (b) the volume when the pressure is 800 × 103 pascals and (c) the pressure when the volume is 1.25 m3 . 6. The power P transmitted by a vee belt drive varies directly as the driving tension T and directly as the belt speed v. A belt transmits 4 kW when the tension is 500 N and the speed is 4 m/s. Determine the power transmitted when the tension is 300 N and the belt speed is 2.5 m/s.

This assignment covers the material contained in Chapters 5 and 6. The marks for each question are shown in brackets at the end of each question. 1. Convert the following binary numbers to decimal form: (a) 1101

(b) 101101.0101

(5)

2. Convert the following decimal number to binary form: (a) 27

(b) 44.1875

(6)

3. Convert the following denary numbers to binary, via octal: (a) 479

(b) 185.2890625

(7)

4. Convert (a) 5F16 into its decimal equivalent (b) 13210 into its hexadecimal equivalent (c) 1101010112 into its hexadecimal equivalent 5. Evaluate 3xy z − 2yz when x = 2 3

6. Simplify the following: (a) (2a + 3b)(x − y) (c)

xy2 + x2 y xy

(b)

4 , 3

y = 2 and z =

(6) 1 2

(3)

√ 8a2 b c3 √ √ (2a2 ) b c

(d) 3x + 4 ÷ 2x + 5 × 2 − 4x

(10)

7. Remove the brackets in the following expressions and simplify: (a) 3b2 − 2(a − a2 b) − b(2a2 − b) (c) 4ab − [3{2(4a − b) + b(2 − a)}] 8. Factorize 3x y + 9xy + 6xy 2

2

3

(b) (2x − y)2 (7) (2)

9. If x is inversely proportional to y and x = 12 when y = 0.4, determine (a) the value of x when y is 3, and (b) the value of y when x = 2 (4)

7 Simple equations

7.1 Expressions, equations and identities (3x − 5) is an example of an algebraic expression, whereas 3x − 5 = 1 is an example of an equation (i.e. it contains an ‘equals’ sign). An equation is simply a statement that two quantities are equal. For example, 1 m = 1000 mm or F = 95 C + 32 or y = mx + c. An identity is a relationship which is true for all values of the unknown, whereas an equation is only true for particular values of the unknown. For example, 3x − 5 = 1 is an equation, since it is only true when x = 2, whereas 3x ≡ 8x − 5x is an identity since it is true for all values of x.

Problem 2.

2x =6 5

The LHS is a fraction and this can be removed by multiplying 2x both sides of the equation by 5. Hence 5 = 5(6) 5 Cancelling gives: 2x = 30 Dividing both sides of the equation by 2 gives: 30 2x = i.e. x = 15 2 2

(Note ‘≡’ means ‘is identical to’). Simple linear equations (or equations of the first degree) are those in which an unknown quantity is raised only to the power 1. To ‘solve an equation’ means ‘to find the value of the unknown’. Any arithmetic operation may be applied to an equation as long as the equality of the equation is maintained.

Solve

Problem 3.

Solve a − 5 = 8

Adding 5 to both sides of the equation gives: a−5+5 = 8+5

7.2 Worked problems on simple equations Problem 1.

Solve the equation 4x = 20

4x 20 = 4 4 (Note that the same operation has been applied to both the lefthand side (LHS) and the right-hand side (RHS) of the equation so the equality has been maintained) Cancelling gives: x = 5, which is the solution to the equation. Solutions to simple equations should always be checked and this is accomplished by substituting the solution into the original equation. In this case, LHS = 4(5) = 20 = RHS. Dividing each side of the equation by 4 gives:

a = 13

i.e.

The result of the above procedure is to move the ‘−5’ from the LHS of the original equation, across the equals sign, to the RHS, but the sign is changed to +. Problem 4.

Solve x + 3 = 7

Subtracting 3 from both sides of the equation gives: x+3−3 = 7−3 i.e.

x=4

The result of the above procedure is to move the ‘+3’ from the LHS of the original equation, across the equals sign, to the RHS, but the sign is changed to −. Thus a term can be moved from

48

Basic Engineering Mathematics

one side of an equation to the other as long as a change in sign is made. Problem 5.

Removing the bracket gives:

3x − 6 = 9

Rearranging gives:

3x = 9 + 6

i.e.

15 3x = 3 3 x= 5

3x = 15

Solve 6x + 1 = 2x + 9

In such equations the terms containing x are grouped on one side of the equation and the remaining terms grouped on the other side of the equation. As in Problems 3 and 4, changing from one side of an equation to the other must be accompanied by a change of sign.

Check:

LHS = 3(5 − 2) = 3(3) = 9 = RHS

Hence the solution x = 5 is correct.

6x + 1 = 2x + 9

Thus since

6x − 2x = 9 − 1

then

Problem 8.

Solve 4(2r − 3) − 2(r − 4) = 3(r − 3) − 1

4x = 8 4x 8 = 4 4 x=2

i.e. Check:

Removing brackets gives: 8r − 12 − 2r + 8 = 3r − 9 − 1

LHS of original equation = 6(2) + 1 = 13 RHS of original equation = 2(2) + 9 = 13

Rearranging gives:

8r − 2r − 3r = −9 − 1 + 12 − 8 3r = −6

i.e.

Hence the solution x = 2 is correct. Problem 6.

Solve 4 − 3p = 2p − 11

In order to keep the p term positive the terms in p are moved to the RHS and the constant terms to the LHS. 4 + 11 = 2p + 3p

Hence

Hence Check:

p=3

LHS = 4 − 3(3) = 4 − 9 = −5 RHS = 2(3) − 11 = 6 − 11 = −5

−3p − 2p = −11 − 4 −5p = −15 −5p −15 = −5 −5 and

LHS = 4(−4 − 3) − 2(−2 − 4) = −28 + 12 = −16 RHS = 3(−2 − 3) − 1 = −15 − 1 = −16

Hence the solution r = −2 is correct.

Exercise 26

Hence the solution p = 3 is correct. If, in this example, the unknown quantities had been grouped initially on the LHS instead of the RHS then:

i.e.

Check:

p = 3, as before

It is often easier, however, to work with positive values where possible.

Further problems on simple equations (Answers on page 273)

Solve the following equations: 1. 2x + 5 = 7 2 3. c − 1 = 3 3 5. 7 − 4p = 2p − 3

2. 8 − 3t = 2 4. 2x −1 = 5x + 11

7. 2a + 6 − 5a = 0

8. 3x − 2 − 5x = 2x − 4

6. 2.6x − 1.3 = 0.9x + 0.4

9. 20d − 3 + 3d = 11d + 5 − 8 10. 2(x − 1) = 4 11. 16 = 4(t + 2) 12. 5( f − 2) − 3(2f + 5) + 15 = 0 13. 2x = 4(x − 3) 14. 6(2 − 3y) − 42 = −2(y − 1) 15. 2(3g − 5) − 5 = 0 16. 4(3x + 1) = 7(x + 4) − 2(x + 5) 17. 10 + 3(r − 7) = 16 − (r + 2)

Probelm 7.

−6 = −2 3

Now try the following exercise

15 = 5p 5p 15 = 5 5 3 = p or

r=

Solve 3(x − 2) = 9

18. 8 + 4(x − 1) − 5(x − 3) = 2(5 − 2x)

Simple equations

7.3 Further worked problems on simple equations Problem 9.

Solve

3 4 = x 5

The lowest common multiple (LCM) of the denominators, i.e. the lowest algebraic expression that both x and 5 will divide into, is 5x. Multiplying both sides by 5x gives: 3 4 5x = 5x x 5 Cancelling gives: 15 4x = 4 4 15 x= 4

3 4 4 3 3 12 4 = = RHS = =3 Check: LHS = = 3 15 15 15 5 3 4 4 (Note that when there is only one fraction on each side of an equation, ‘cross-multiplication’ can be applied.) In this example, 3 4 if = then (3)(5) = 4x, which is a quicker way of arriving at x 5 equation (1) above. Problem 10.

or 3

Solve

1 3y 2y 3 + +5= − 5 4 20 2

The LCM of the denominators is 20. Multiplying each term by 20 gives: 2y 3 1 3y 20 + 20 + 20(5) = 20 − 20 5 4 20 2 Cancelling gives: 4(2y) + 5(3) + 100 = 1 − 10(3y) i.e.

8y + 15 + 100 = 1 − 30y

Rearranging gives: 8y + 30y = 1 − 15 − 100 38y = −114 −114 y= = −3 38 Check:

−6 3 2(−3) 3 + +5= + +5 5 4 5 4 11 −9 +5=4 = 20 20 3(−3) 1 9 11 1 − = + =4 RHS = 20 2 20 2 20 LHS =

Hence the solution y = −3 is correct.

Solve

4 3 = t − 2 3t + 4

By ‘cross-multiplication’:

3(3t + 4) = 4(t − 2)

Removing brackets gives:

9t + 12 = 4t − 8

Rearranging gives:

9t − 4t = −8 − 12 5t = −20

i.e.

t= Check:

15 = 4x

i.e.

Problem 11.

49

−20 = −4 5

3 1 3 = =− −4 − 2 −6 2 4 4 1 4 = = =− RHS = 3(−4) + 4 −12 + 4 −8 2 LHS =

Hence the solution t = −4 is correct. Problem 12.

Solve

√ x=2

√ [ x√= 2 is not a ‘simple equation’ since the power of x is 12 i.e. x = x(1/2) ; however, it is included here since it occurs often in practise]. Wherever square root signs are involved with the unknown quantity, both sides of the equation must be squared. Hence √ ( x)2 = (2)2 i.e.

x=4

Problem 13.

√ Solve 2 2 = 8

To avoid possible errors it is usually best to arrange the term containing the square root on its own. Thus √ 2 d 8 = 2 2 √ d =4 i.e. Squaring both sides gives: d = 16, which may be checked in the original equation.

Problem 14.

√

b+3 Solve =2 √ b

To remove the fraction each term is multiplied by √

√ √ b+3 b = b(2) √ b

√ b. Hence:

50

Basic Engineering Mathematics

√ √ b+3=2 b √ √ √ 3=2 b− b= b

Cancelling gives: Rearranging gives:

Squaring both sides gives: 9 = b √ 9 + 3 3+3 6 = = 2 = RHS Check: LHS = √ = 3 3 9 Problem 15.

Solve x2 = 25

This problem involves a square term and thus is not a simple equation (it is, in fact, a quadratic equation). However the solution of such an equation is often required and is therefore included here for completeness. Whenever a square of the unknown is involved, the square root of both sides of the equation is taken. Hence √ √ x2 = 25 x=5

i.e.

However, x = −5 is also a solution of the equation because (−5) × (−5) = +25 Therefore, whenever the square root of a number is required there are always two answers, one positive, the other negative. The solution of x2 = 25 is thus written as x = ±5. Problem 16.

Solve

15 2 = 4t 2 3

‘Cross-multiplying’ gives:

5.

1 1 1 (3m − 6) − (5m + 4) + (2m − 9) = −3 3 4 5

6.

x x − =2 3 5

7. 1 −

8.

2 3 = a 8

9.

7 1 1 = + 3n 4n 24

10.

x+3 x−3 = +2 4 5

11.

3t 6−t 2t 3 = + − 20 12 15 2

12.

7 5−y y + = 5 20 4

13.

v−2 1 = 2v − 3 3

3 2 = a − 3 2a + 1 √ 16. 3 t = 9 14.

3 18. 4 = +3 a

x −1 20. 10 = 5 2 y+2 1 = 22. y−2 2 24.

x x+6 x+3 − = 4 5 2 √ 17. 2 y = 5 15.

19.

√ 3 x √ = −6 1− x

21. 16 =

23.

t2 9

6 2a = a 3

8 11 =5+ 2 2 x

15(3) = 2(4t 2 )

i.e.

45 = 8t 2

i.e.

45 = t2 8 t 2 = 5.625

√ Hence t = 5.625 = ±2.372, correct to 4 significant figures. Now try the following exercise

7.4 Practical problems involving simple equations Problem 17. A copper wire has a length l of 1.5 km, a resistance R of 5 and a resistivity of 17.2 × 10−6 mm. Find the cross-sectional area, a, of the wire, given that R = ρl/a

Since

Exercise 27 Further problems on simple equations (Answers on page 273) Solve the following equations: 1 d+3=4 5 2 5 3 2. 2 + y = 1 + y + 4 3 6 1.

1 1 (2x − 1) + 3 = 4 2 1 2 1 =0 4. (2f − 3) + ( f − 4) + 5 6 15 3.

y y y =3+ − 3 3 6

R = ρl/a then 5=

(1.72 × 10−6 mm)(1500 × 103 mm) a

From the units given, a is measured in mm2 . Thus and

5a = 17.2 × 10−6 × 1500 × 103 17.2 × 10−6 × 1500 × 103 5 17.2 × 1500 × 103 17.2 × 15 = = 5.16 = 106 × 5 10 × 5

a=

Hence the cross-sectional area of the wire is 5.16 mm2 .

Simple equations

Problem 18. A rectangular box with square ends has its length 15 cm greater than its breadth and the total length of its edges is 2.04 m. Find the width of the box and its volume. Let x cm = width = height of box. Then the length of the box is (x + 15) cm. The length of the edges of the box is 2(4x) + 4(x + 15) cm. Hence

204 − 60 = 12x

and

Problem 21. When three resistors in an electrical circuit are connected in parallel the total resistance RT is given by: 1 1 1 1 = + + R T R1 R2 R3 Find the total resistance when R1 = 5 , R2 = 10 and R3 = 30 . 1 1 1 1 + = + RT 5 10 30 6 + 3 + 1 10 1 = = = 30 30 3 Taking the reciprocal of both sides gives: RT = 3

204 = 2(4x) + 4(x + 15) 204 = 8x + 4x + 60

i.e.

51

144 = 12x x = 12 cm

Alternatively, if ators is 30 RT

Hence the width of the box is 12 cm. Volume of box = length × width × height = (x + 15)(x)(x) = (27)(12)(12)

1 1 1 1 + the LCM of the denomin= + RT 5 10 30

Hence 30RT

= 3888 cm3 Problem 19. The temperature coefficient of resistance α may be calculated from the formula Rt = R0 (1 + αt). Find α given Rt = 0.928, R0 = 0.8 and t = 40

1 RT

1 1 + 30RT 5 10 1 + 30RT 30

= 30RT

Cancelling gives: 30 = 6RT + 3RT + RT

Since

Rt = R0 (1 + αt)

then

0.928 = 0.8[1 + α(40)] 0.928 = 0.8 + (0.8)(α)(40)

0.928 − 0.8 = 32α

30 = 10RT 30 = 3 , as above RT = 10

0.128 = 32α α=

Hence

0.128 = 0.004 32

Problem 20. The distance s metres travelled in time t seconds is given by the formula s = ut + 12 at 2 , where u is the initial velocity in m/s and a is the acceleration in m/s2 . Find the acceleration of the body if it travels 168 m in 6 s, with an initial velocity of 10 m/s. s = ut + 12 at 2 , and s = 168, u = 10 and t = 6 Hence

168 = (10)(6) + 12 a(6)2 168 = 60 + 18a 168 − 60 = 18a 108 = 18a a=

108 =6 18

Hence the acceleration of the body is 6 m/s2 .

Now try the following exercise Exercise 28

Practical problems involving simple equations (Answers on page 273)

1. A formula used for calculating resistance of a cable is R = (ρl)/a. Given R = 1.25, l = 2500 and a = 2 × 10−4 find the value of ρ. 2. Force F newtons is given by F = ma, where m is the mass in kilograms and a is the acceleration in metres per second squared. Find the acceleration when a force of 4 kN is applied to a mass of 500 kg. 3. PV = mRT is the characteristic gas equation. Find the value of m when P = 100 × 103 , V = 3.00, R = 288 and T = 300 4. When three resistors R1 , R2 and R3 are connected in parallel the total resistance RT is determined from 1 1 1 1 = + + RT R1 R2 R3 (a) Find the total resistance when R1 = 3 , R2 = 6 and R3 = 18 .

52

Basic Engineering Mathematics

(b) Find the value of R3 given that RT = 3 , R1 = 5 and R2 = 10 . 5. Five pens and two rulers cost 94p. If a ruler costs 5p more than a pen, find the cost of each. 6. Ohm’s law may be represented by I = V /R, where I is the current in amperes, V is the voltage in volts and R is the resistance in ohms. A soldering iron takes a current of 0.30 A from a 240 V supply. Find the resistance of the element.

Problem 24. A painter is paid £4.20 per hour for a basic 36 hour week, and overtime is paid at one and a third times this rate. Determine how many hours the painter has to work in a week to earn £212.80 Basic rate per hour = £4.20; 1 overtime rate per hour = 1 × £4.20 = £5.60 3 Let the number of overtime hours worked = x Then

(36)(4.20) + (x)(5.60) = 212.80 151.20 + 5.60x = 212.80

7.5 Further practical problems involving simple equations Problem 22. The extension x m of an aluminium tie bar of length l m and cross-sectional area A m2 when carrying a load of F newtons is given by the modulus of elasticity E = Fl/Ax. Find the extension of the tie bar (in mm) if E = 70 × 109 N/m2 , F = 20 × 106 N, A = 0.1 m2 and l = 1.4 m.

E = Fl/Ax,

70 × 109

hence

(20 ×106 N)(1.4 m) N = 2 m (0.1 m2 )(x) (the unit of x is thus metres)

70 × 109 × 0.1 × x = 20 × 106 × 1.4 x=

20 × 106 × 1.4 70 × 109 × 0.1

x=

2 × 1.4 2 × 1.4 m= × 1000 mm 7 × 100 7 × 100

Hence the total number of hours worked is 36 + 11, i.e. 47 hours. Problem 25. A formula relating initial and final states of pressures, P1 and P2 , volumes V1 and V2 , and absolute temP1 V1 P2 V2 peratures, T1 and T2 , of an ideal gas is = . Find T1 T2 3 the value of P2 given P1 = 100 × 10 , V1 = 1.0, V2 = 0.266, T1 = 423 and T2 = 293 P 1 V1 P 2 V2 (100 × 103 )(1.0) P2 (0.266) = = then T1 T2 423 293

‘Cross-multiplying’ gives: (100 × 103 )(1.0)(293) = P2 (0.266)(423)

Hence the extension of the tie bar, x = 4 mm. Problem 23. Power in a d.c. circuit is given by V2 where V is the supply voltage and R is the P= R circuit resistance. Find the supply voltage if the circuit resistance is 1.25 and the power measured is 320 W.

Since

P=

V2 R

then

320 =

V2 1.25

(320)(1.25) = V 2 i.e. Supply voltage,

V 2 = 400 √ V = 400 = ±20 V

61.60 = 11 5.60

Thus 11 hours overtime would have to be worked to earn £212.80 per week.

Since

Cancelling gives: x=

5.60x = 212.80 − 151.20 = 61.60

P2 =

(100 × 103 )(1.0)(293) (0.266)(423)

Hence P2 = 260 × 103 or 2.6 × 105 Problem 26. The stress f in a material of a thick cylinder f +p D . Calculate the stress, can be obtained from = d f −p given that D = 21.5, d = 10.75 and p = 1800

Since

D = d

f +p f −p

21.5 = then 10.75

i.e.

2=

f + 1800 f − 1800 f + 1800 f − 1800

Simple equations

Squaring both sides gives: f + 1800 4= f − 1800 4(f − 1800) = f + 1800 4f − 7200 = f + 1800 4f − f = 1800 + 7200 3f = 9000 9000 = 3000 3 stress, f = 3000 f=

Hence

Problem 27. 12 workmen employed on a building site earn between them a total of £2415 per week. Labourers are paid £175 per week and craftsmen are paid £220 per week. How many craftsmen and how many labourers are employed? Let the number of craftsmen be c. The number of labourers is therefore (12 − c) The wage bill equation is: 220c + 175(12 − c) = 2415 220c + 2100 − 175c = 2415 220c − 175c = 2415 − 2100 45c = 315 315 c= =7 45 Hence there are 7 craftsmen and (12 − 7), i.e. 5 labourers on the site.

53

Now try the following exercise Exercise 29

Practical problems involving simple equation (Answers on page 273)

1. A rectangle has a length of 20 cm and a width b cm. When its width is reduced by 4 cm its area becomes 160 cm2 . Find the original width and area of the rectangle. 2. Given R2 = R1 (1 + αt), find α given R1 = 5.0, R2 = 6.03 and t = 51.5 3. If v 2 = u2 + 2as, find u given v = 24, a = −40 and s = 4.05 4. The relationship between the temperature on a Fahrenheit scale and that on a Celsius scale is given by F = 95 C + 32. Express 113◦ F in degrees Celsius. 5. If t = 2π (w/Sg), find the value of S given w = 1.219, g = 9.81 and t = 0.3132 6. Two joiners and five mates earn £1216 between them for a particular job. If a joiner earns £48 more than a mate, calculate the earnings for a joiner and for a mate. 7. An alloy contains 60% by weight of copper, the remainder being zinc. How much copper must be mixed with 50 kg of this alloy to give an alloy containing 75% copper? 8. A rectangular laboratory has a length equal to one and a half times its width and a perimeter of 40 m. Find its length and width. 9. Applying the principle of moments to a beam results in the following equation: F × 3 = (5 − F) × 7 where F is the force in newtons. Determine the value of F.

8 Transposition of formulae

8.1 Introduction to transposition of formulae

Problem 2. If a + b = w − x + y, express x as the subject.

When a symbol other than the subject is required to be calculated it is usual to rearrange the formula to make a new subject. This rearranging process is called transposing the formula or transposition. The rules used for transposition of formulae are the same as those used for the solution of simple equations (see Chapter 7) — basically, that the equality of an equation must be maintained.

Problem 1. Transpose p = q + r + s to make r the subject. The aim is to obtain r on its own on the left-hand side (LHS) of the equation. Changing the equation around so that r is on the LHS gives:

q + r + s − q − s= p − q − s r= p−q−s

Multiplying both sides by −1 gives: (−1)(−x) = (−1)(a + b − w − y) i.e.

x = −a−b+w+y

Problem 3. Transpose v = f λ to make λ the subject. Rearranging gives: f λ = v Dividing both sides by f gives:

fλ v = , f f

i.e. λ =

v f

Problem 4. When a body falls freely through a height h, the velocity v is given by v 2 = 2gh. Express this formula with h as the subject.

q + r + s − (q + s) = p − (q + s)

i.e.

−x =a+b−w−y

(1)

Subtracting (q + s) from both sides of the equation gives:

Thus

w − x + y = a + b and

The result of multiplying each side of the equation by −1 is to change all the signs in the equation. It is conventional to express answers with positive quantities first. Hence rather than x = −a − b + w + y, x = w + y − a − b, since the order of terms connected by + and − signs is immaterial.

8.2 Worked problems on transposition of formulae

q+r+s=p

Rearranging gives:

Rearranging gives: 2gh = v 2 (2) Dividing both sides by 2g gives:

It is shown with simple equations, that a quantity can be moved from one side of an equation to the other with an appropriate change of sign. Thus equation (2) follows immediately from equation (1) above.

Problem 5.

If I =

2gh v 2 = , 2g 2g

i.e.

h=

v2 2g

V , rearrange to make V the subject. R

Transposition of formulae

V =I R Multiplying both sides by R gives: V R = R(I ) R

Now try the following exercise

Rearranging gives:

Hence

Exercise 30

Further problems on transposition of formulae (Answers on page 273)

Make the symbol indicated the subject of each of the formulae shown and express each in its simplest form.

V = IR

Problem 6. Transpose a =

55

F for m m

F Rearranging gives: = a m Multiplying both sides by m gives: F m = m(a) m

i.e. F = ma

Rearranging gives: Dividing both sides by a gives: i.e.

ma = F ma F = a a F m= a

Problem 7. Rearrange the formula R = (i) a the subject, and (ii) l the subject.

ρl =R a Multiplying both sides by a gives: ρl = a(R) i.e. ρl = aR a a

(i) Rearranging gives:

Rearranging gives: aR = ρl Dividing both sides by R gives: aR ρl = R R ρl i.e. a= R ρl =R (ii) a Multiplying both sides by a gives: ρl = aR Dividing both sides by ρ gives: ρl aR = ρ ρ aR i.e. l= ρ

ρl to make a

1. a + b = c − d − e

(d)

2. x + 3y = t

(y)

3. c = 2πr

(r)

4. y = mx + c

(x)

5. I = PRT

(T)

E 6. I = R

(R)

7. S =

a 1−r

(r)

9 8. F = C + 32 5

(C)

8.3 Further worked problems on transposition of formulae Problem 8. Transpose the formula v = u + the subject. ft = v and m Multiplying each side by m gives:

Rearranging gives: u +

m

ft = m(v − u) m

ft , to make f m

ft =v−u m

ft = m(v − u)

i.e.

Dividing both sides by t gives: m ft = (v − u) t t

i.e.

f =

m (v − u) t

Problem 9. The final length, l2 of a piece of wire heated through θ ◦ C is given by the formula l2 = l1 (1 + αθ ). Make the coefficient of expansion, α, the subject. Rearranging gives: Removing the bracket gives: Rearranging gives:

l1 (1 + αθ ) = l2 l1 + l1 αθ = l2 l1 αθ = l2 − l1

56

Basic Engineering Mathematics

Dividing both sides by l1 θ gives: l2 − l 1 l1 αθ = l1 θ l1 θ

Problem 13. Given t = 2π

l2 − l 1 i.e. α = l1 θ

Problem 10. A formula for the distance moved by a body is given by s = 12 (v + u)t. Rearrange the formula to make u the subject. 1 (v + u)t = s 2 (v + u)t = 2s

Rearranging gives: Multiplying both sides by 2 gives:

(v + u)t 2s = t t 2s v+u = t 2s − υt 2s − υ or u = u= t t

Whenever the prospective new subject is within a square root sign, it is best to isolate that term on the LHS and then to square both sides of the equation. l =t Rearranging gives: 2π g t l Dividing both sides by 2π gives: = g 2π

Dividing both sides by t gives: i.e. Hence

Problem 11. A formula for kinetic energy is k = 12 mv 2 . Transpose the formula to make v the subject.

l , find g in terms of t, l and π. g

Squaring both sides gives:

l = g

2

t 2π

=

t2 4π 2

Cross-multiplying, i.e. multiplying each term by 4π 2 g, gives: 4π 2 l = gt 2 gt 2 = 4π 2 l

or

gt 2 4π 2 l = 2 2 t t 4π 2 l g= 2 t

Dividing both sides by t 2 gives: i.e.

Rearranging gives: 12 mv 2 = k Whenever the prospective new subject is a squared term, that term is isolated on the LHS, and then the square root of both sides of the equation is taken. Multiplying both sides by 2 gives:

mv 2 = 2k

2k mv 2 = m m 2k 2 i.e. v = m Taking the square root of both sides gives: √ 2k 2 v = m 2k i.e. v= m Dividing both sides by m gives:

Problem √ 14. The impedance of an a.c. circuit is given by Z = R2 + X 2 . Make the reactance, X , the subject. √ R2 + X 2 = Z

Rearranging gives: Squaring both sides gives:

R2 + X 2 = Z 2 X 2 = Z 2 − R2

Rearranging gives:

Taking the square root of both sides gives: √ X = Z 2 − R2 Problem 15. The volume V of a hemisphere is given by V = 23 πr 3 . Find r in terms of V . 2 3 πr = V 3 Multiplying both sides by 3 gives: 2π r 3 = 3V

Rearranging gives: Problem 12. In a right angled triangle having sides x, y and hypotenuse z, Pythagoras’ theorem states z 2 = x2 + y2 . Transpose the formula to find x. Rearranging gives: and

x2 + y2 = z 2 x2 = z 2 − y2

Taking the square root of both sides gives: x = z 2 − y2

Dividing both sides by 2π gives: 3V 2πr 3 = 2π 2π

i.e.

r3 =

3V 2π

Taking the cube root of both sides gives: √ 3V 3V 3 r3 = 3 i.e. r = 3 2π 2π

Transposition of formulae

Dividing both sides by (x + y) gives:

Now try the following exercise

a2 (x + y) rp = (x + y) (x + y)

Exercise 31 Further problems on transposition of formulae (Answers on page 273) Make the symbol indicated the subject of each of the formulae shown and express each in its simplest form. 1. y =

λ(x − d) d

(x)

2. A =

3(F − f ) L

(f)

3. y =

Ml 2 8EI

(E)

4. R = R0 (1 + αt)

(t)

5.

1 1 1 = + R R1 R2

(R2 )

E−e R+r

(R)

6. I =

7. y = 4ab2 c2 2

(b)

2

a b + =1 x2 y2 l 9. t = 2π g 8.

10. v 2 = u2 + 2as πR2 θ 360 a+x 12. N = y 11. A =

13. Z = R2 + (2π f L)2

(x)

(l) (u) (R)

(a) (L)

8.4 Harder worked problems on transposition of formulae a2 x + a2 y to Problem 16. Transpose the formula p = r make a the subject. a2 x + a2 y =p r

Multiplying both sides by r gives:

a2 x + a2 y = rp

Factorizing the LHS gives:

a2 (x + y) = rp

a2 =

rp (x + y)

Taking the square root of both sides gives: rp a= x+y Problem 17. Make b the subject of the formula x−y a= √ bd + be x−y

=a bd + be √ Multiplying both sides by bd + be gives: √ x − y = a bd + be √ or a bd + be = x − y √ x−y Dividing both sides by a gives: bd + be = a x−y 2 Squaring both sides gives: bd + be = a x−y 2 Factorizing the LHS gives: b(d + e) = a x−y 2 a Dividing both sides by (d + e) gives: b= (d + e) Rearranging gives: √

b=

i.e.

(x − y)2 a2 (d + e)

Problem 18. If cd = 3d + e − ad, express d in terms of a, c and e Rearranging to obtain the terms in d on the LHS gives: cd − 3d + ad = e Factorizing the LHS gives: d(c − 3 + a) = e Dividing both sides by (c − 3 + a) gives: d=

Rearranging gives:

i.e.

e c−3+a

Problem 19. If a = formula.

b , make b the subject of the 1+b

57

58

Basic Engineering Mathematics

b =a 1+b

Rearranging gives: Multiplying both sides by (1 + b) gives:

b = a(1 + b)

Removing the bracket gives:

b = a + ab

Rearranging to obtain terms in b on the LHS gives: b − ab = a

Dividing both sides by ( p − y − t) gives: q2 =

Taking the square root of both sides gives: r( y + t) q= p−y−t

b(1 − a) = a

Factorizing the LHS gives:

a b= 1−a

Dividing both sides by (1 − a) gives:

Problem 20. Transpose the formula V = the subject.

Er to make r R+r

D = Problem 22. Given that d terms of D, d and f .

f +p , express p in f −p

D f +p = f −p d f +p D2 = 2 f −p d

Rearranging gives:

Squaring both sides gives: Er =V R+r

Rearranging gives:

r( y + t) ( p − y − t)

Cross-multiplying, i.e. multiplying each term by d 2 ( f − p),

Multiplying both sides by (R + r) gives:

Er = V (R + r)

gives:

d 2 ( f + p) = D2 ( f − p)

Removing the bracket gives:

Er = VR + Vr

Removing brackets gives:

d 2 f + d 2 p = D2 f − D2 p

Rearranging to obtain terms in r on the LHS gives:

Rearranging, to obtain terms in p on the LHS gives:

Er − Vr = VR r(E − V ) = VR

Factorizing gives: Dividing both sides by (E − V ) gives:

r=

Problem 21. Transpose the formula y = to make q the subject

Rearranging gives: and

VR E−V

pq2 −t r + q2

Factorizing gives:

pq2 −t = y r + q2 pq2 = y+t r + q2

pq2 = ry + rt + q2 y + q2 t

Factorizing gives:

q2 ( p − y − t) = r( y + t)

p=

f (D2 − d 2 ) (d 2 − D2 )

Now try the following exercise Further problems on transposition of formulae (Answers on page 274)

Make the symbol indicated the subject of each of the formulae shown in Problems 1 to 7, and express each in its simplest form. 1. y =

a2 m − a2 n x

2. M = π (R4 − r 4 )

(a) (R)

r 3+r

(r)

4. m =

µL L + rCR

(L)

5. a2 =

b2 − c 2 b2

(b)

3. x + y =

Rearranging to obtain terms in q on the LHS gives: pq2 − q2 y − q2 t = ry + rt

p(d 2 + D2 ) = f (D2 − d 2 )

Dividing both sides by (d 2 + D2 ) gives:

Exercise 32

Multiplying both sides by (r + q2 ) gives: pq2 = (r + q2 )( y + t) Removing brackets gives:

d 2 p + D2 p = D2 f − d 2 f

Transposition of formulae

x 1 + r2 = y 1 − r2 a + 2b p 7. = q a − 2b 6.

(r)

Assignment 4

(b)

This assignment covers the material contained in Chapters 7 and 8. The marks for each question are shown in brackets at the end of each question.

8. A formula for the focal length, f , of a convex lens is 1 1 1 = + . Transpose the formula to make v the subject f u v and evaluate v when f = 5 and u = 6. 9. The quantity of heat, Q, is given by the formula Q = mc(t2 − t1 ). Make t2 the subject of the formula and evaluate t2 when m = 10, t1 = 15, c = 4 and Q = 1600. 10. The velocity, v, of water in a pipe appears in the formula 0.03Lv 2 . Express v as the subject of the formula h= 2dg and evaluate v when h = 0.712, L = 150, d = 0.30 and g = 9.81. 11. The sag S at the centre of a wire is given by the formula: 3d(l − d) . Make l the subject of the formula S= 8 and evaluate l when d = 1.75 and S = 0.80. 12. In an electrical alternating current circuit the impedance 1 2 R2 + ωL − . Z is given by: Z = ωC Transpose the formula to make C the subject and hence evaluate C when Z = 130, R = 120, ω = 314 and L = 0.32 13. An approximate relationship between the number of teeth, T , on a milling cutter, the diameter of cutter, D, 12.5D and the depth of cut, d, is given by: T = D + 4d Determine the value of D when T = 10 and d = 4 mm. 14. Make λ, the wavelength of X-rays, the subject of the following formula: √ CZ 4 λ5 n µ = a ρ

1. Solve the following equations: (a) 3t − 2 = 5t − 4 (b) 4(k − 1) − 2(3k + 2) + 14 = 0 a 2a (c) − =1 5 2 √ (d) 3 y = 2 s+1 (e) =2 s−1

(20)

2. Distance s = ut + 12 at 2 . Find the value of acceleration a when s = 17.5, time t = 2.5 and u = 10 (4) 3. A rectangular football pitch has its length equal to twice its width and a perimeter of 360 m. Find its length and width. (4) 4. Transpose the following equations: (a) y = mx + c

for m

2(y − z) for z t 1 1 1 (c) = + for RA RT RA R B

(b) x =

(d) x2 − y2 = 3ab for y p−q for q (e) K = 1 + pq

(18)

5. The passage of sound waves through walls is governed by the equation: v=

K + 43 G ρ

Make the shear modulus G the subject of the formula. (4)

59

9 Simultaneous equations

9.1 Introduction to simultaneous equations

Substituting this expression for x into equation (2) gives:

Only one equation is necessary when finding the value of a single unknown quantity (as with simple equations in Chapter 7). However, when an equation contains two unknown quantities it has an infinite number of solutions. When two equations are available connecting the same two unknown values then a unique solution is possible. Similarly, for three unknown quantities it is necessary to have three equations in order to solve for a particular value of each of the unknown quantities, and so on. Equations which have to be solved together to find the unique values of the unknown quantities, which are true for each of the equations, are called simultaneous equations. Two methods of solving simultaneous equations analytically are:

This is now a simple equation in y.

(a) by substitution, and (b) by elimination.

Thus x = 3 and y = −2 is the solution to the simultaneous equations.

4(−1 − 2y) − 3y = 18

(A graphical solution of simultaneous equations is shown in Chapter 13.)

Removing the bracket gives: −4 − 8y − 3y = 18 −11y = 18 + 4 = 22 22 y= = −2 −11 Substituting y = −2 into equation (1) gives: x + 2( − 2) = −1 x − 4 = −1 x = −1 + 4 = 3

(Check: In equation (2), since x = 3 and y = −2, LHS = 4(3) − 3( − 2) = 12 + 6 = 18 = RHS.) (b) By elimination

9.2 Worked problems on simultaneous equations in two unknowns Problem 1. Solve the following equations for x and y, (a) by substitution, and (b) by elimination: x + 2y = −1

(1)

4x − 3y = 18

(2)

(a) By substitution From equation (1): x = −1 − 2y

x + 2y = −1

(1)

4x − 3y = 18

(2)

If equation (1) is multiplied throughout by 4 the coefficient of x will be the same as in equation (2), giving: 4x + 8y = −4

(3)

Subtracting equation (3) from equation (2) gives: 4x − 3y = 18

(2)

4x + 8y = −4 ———–——— 0 − 11y = 22 ———–———

(3)

Simultaneous equations

Hence y =

22 = −2 −11

61

(Note +8y − −15y = 8y + 15y = 23y and 10 − −36 = 10 + 36 = 46. Alternatively, ‘change the signs of the bottom line and add’.)

(Note, in the above subtraction, 18 − −4 = 18 + 4 = 22)

Substituting y = 2 in equation (1) gives:

Substituting y = −2 into either equation (1) or equation (2) will give x = 3 as in method (a). The solution x = 3, y = −2 is the only pair of values that satisfies both of the original equations. Problem 2. Solve, by a substitution method, the simultaneous equations 3x − 2y = 12

(1)

x + 3y = −7

(2)

From equation (2), x = −7 − 3y

3x + 4(2) = 5 from which 3x = 5 − 8 = −3 x = −1

and

Checking in equation (2), left-hand side = 2(−1) − 5(2) = −2 − 10 = −12 = right-hand side. Hence x = −1 and y = 2 is the solution of the simultaneous equations. The elimination method is the most common method of solving simultaneous equations.

Problem 4. Solve

Substituting for x in equation (1) gives: 3(−7 − 3y) − 2y = 12 −21 − 9y − 2y = 12

i.e.

−11y = 12 + 21 = 33 Hence

x + 3(−3) = −7

Hence

(1) (2)

When equation (1) is multiplied by 5 and equation (2) by 2 the coefficients of y in each equation are numerically the same, i.e. 10, but are of opposite sign.

33 = −3 y= −11

Substituting y = −3 in equation (2) gives:

i.e.

7x − 2y = 26 6x + 5y = 29

x − 9 = −7

5 × equation (1) gives:

35x − 10y = 130

(3)

2 × equation (2) gives:

12x + 10y = 58

(4)

Adding equation (3) and (4) gives:

47x + 0 = 188

x = −7 + 9 = 2

Thus x = 2, y = −3 is the solution of the simultaneous equations. (Such solutions should always be checked by substituting values into each of the original two equations.) Problem 3. Use an elimination method to solve the simultaneous equations

188 =4 47 [Note that when the signs of common coefficients are different the two equations are added, and when the signs of common coefficients are the same the two equations are subtracted (as in Problems 1 and 3)]

Hence x =

Substituting x = 4 in equation (1) gives:

3x + 4y = 5

(1)

7(4) − 2y = 26

2x − 5y = −12

(2)

28 − 2y = 26 28 − 26 = 2y 2 = 72y

If equation (1) is multiplied throughout by 2 and equation (2) by 3, then the coefficient of x will be the same in the newly formed equations. Thus

Hence y = 1

2 × equation (1) gives:

6x + 8y = 10

(3)

Checking, by substituting x = 4 and y = 1 in equation (2), gives:

3 × equation (2) gives:

6x − 15y = −36

(4)

Equation (3) − equation (4) gives:

0 + 23y = 46

i.e.

46 =2 y= 23

LHS = 6(4) + 5(1) = 24 + 5 = 29 = RHS Thus the solution is x = 4, y = 1, since these values maintain the equality when substituted in both equations.

62

Basic Engineering Mathematics

Now try the following exercise

Problem 6. Solve 5 x + =y 8 2 y 13 − = 3x 3

Exercise 33 Further problems on simultaneous equations (Answers on page 274) Solve the following simultaneous equations and verify the results. 1. a + b = 7

2. 2x + 5y = 7

a−b=3

x + 3y = 4

3. 3s + 2t = 12 5. 5m − 3n = 11

2x + 5y = −4 6. 8a − 3b = 51

3m + n = 8 7. 5x = 2y

3a + 4b = 14

x + 20 = 8y

i.e.

8. 5c = 1 − 3d

3x + 7y = 41

(2)

Whenever fractions are involved in simultaneous equations it is usual to firstly remove them. Thus, multiplying equation (1) by 8 gives: x

5 +8 = 8y 8 8 2

4. 3x − 2y = 13

4s − t = 5

(1)

(3)

Multiplying equation (2) by 3 gives:

2d + c + 4 = 0

39 − y = 9x

(4)

Rearranging equations (3) and (4) gives:

9.3 Further worked problems on simultaneous equations

(5)

9x + y = 39

(6)

Multiplying equation (6) by 8 gives:

Problem 5. Solve 3p = 2q

(1)

4p + q + 11 = 0

(2)

72x + 8y = 312

3p − 2q = 0 4p + q = −11

73x + 0 = 292 292 =4 x= 73

(3) (4)

Substituting x = 4 into equation (5) gives: 4 − 8y = −20

Multiplying equation (4) by 2 gives: 8p + 2q = −22

4 + 20 = 8y

(5)

24 = 8y 24 =3 y= 8

Adding equations (3) and (5) gives: 11p + 0 = −22 −22 = −2 p= 11 Substituting p = −2 into equation (1) gives: 3(−2) = 2q −6 = 2q −6 = −3 q= 2 Checking, by substituting p = −2 and q = −3 into equation (2) gives: LHS = 4(−2) + (−3) + 11 = −8 − 3 + 11 = 0 = RHS

(7)

Adding equations (5) and (7) gives:

Rearranging gives:

Hence the solution is p = −2, q = −3

x − 8y = −20

Checking: substituting x = 4, y = 3 in the original equations, gives: 4 5 1 1 + = + 2 = 3 = y = RHS 8 2 2 2 3 Equation (2) : LHS = 13 − = 13 − 1 = 12 3 RHS = 3x = 3(4) = 12 Equation (1) :

LHS =

Hence the solution is x = 4, y = 3 Problem 7. Solve 2.5x + 0.75 − 3y = 0 1.6x = 1.08 − 1.2y

Simultaneous equations

It is often easier to remove decimal fractions. Thus multiplying equations (1) and (2) by 100 gives: 250x + 75 − 300y = 0

(1)

160x = 108 − 120y

(2)

5.

7.

Rearranging gives:

x 2y 49 + = 5 3 15 3x y 5 − + =0 7 2 7 1.5x − 2.2y = −18 2.4x + 0.6y = 33

250x − 300y = −75

(3)

160x + 120y = 108

(4)

Multiplying equation (3) by 2 gives: 500x − 600y = −150

63

u 12 v 25 u+ − =0 4 2

6.

v−1=

8.

3b − 2.5a = 0.45 1.6a + 0.8b = 0.8

9.4 More difficult worked problems on simultaneous equations

(5) Problem 8. Solve

Multiplying equation (4) by 5 gives: 800x + 600y = 540

2 3 + =7 y x 1 4 − = −2 x y

(6)

Adding equations (5) and (6) gives: 1300x + 0 = 390 390 39 3 x= = = = 0.3 1300 130 10 Substituting x = 0.3 into equation (1) gives: 250(0.3) + 75 − 300y = 0 75 + 75 = 300y 150 = 300y 150 = 0.5 y= 300

RHS = 108 − 120(0.5) = 108 − 60 = 48

(2)

In this type of equation the solution is easier if a substitution is initially made. Let

1 1 = a and = b x y

Thus equation (1) becomes: and equation (2) becomes: Multiplying equation (4) by 2 gives:

2a + 3b = 7

(3)

a − 4b = −2

(4)

2a − 8b = −4

(5)

Subtracting equation (5) from equation (3) gives:

Checking x = 0.3, y = 0.5 in equation (2) gives: LHS = 160(0.3) = 48

(1)

0 + 11b = 11 b=1

i.e. Substituting b = 1 in equation (3) gives:

Hence the solution is x = 0.3, y = 0.5

2a + 3 = 7 2a = 7 − 3 = 4

Now try the following exercise Exercise 34 Further problems on simultaneous equations (Answers on page 274) Solve the following simultaneous equations and verify the the results. x y + =4 1. 7p + 11 + 2q = 0 2. 2 3 x y − =0 −1 = 3q − 5p 6 9 3 a − 7 = −2b 4. s − 2t = 8 3. 2 2 s 2 + 3y = −2 12 = 5a + b 3 4

a=2

i.e.

Checking, substituting a = 2 and b = 1 in equation (4) gives: LHS = 2 − 4(1) = 2 − 4 = −2 = RHS Hence a = 2 and b = 1 However, since and since

1 1 1 = a then x = = a 2 x 1 1 1 = b then y = = = 1 y b 1

Hence the solution is x = 12 , y = 1, which may be checked in the original equations.

64

Basic Engineering Mathematics

Problem 9. Solve 3 1 + =4 2a 5b 4 1 + = 10.5 a 2b

Let

(1)

To eliminate fractions, both sides of equation (1) are multiplied by 27(x + y) giving: 1 4 27(x + y) = 27(x + y) x+y 27

(2)

27(1) = 4(x + y)

i.e.

27 = 4x + 4y

1 1 = x and = y a b 3 x + y=4 2 5 1 4x + y = 10.5 2

then

Similarly, in equation (2):

33 = 4(2x − y)

i.e.

33 = 8x − 4y

(3)

(3) (4)

To remove fractions, equation (3) is multiplied by 10 giving: x

3 y = 10(4) + 10 10 2 5

(4)

Equation (3) + equation (4) gives: 60 =5 12 Substituting x = 5 in equation (3) gives: 60 = 12x,

i.e. x =

27 = 4(5) + 4y 5x + 6y = 40

i.e.

(5)

Multiplying equation (4) by 2 gives: 8x + y = 21

(6)

(7) Problem 11.

Substituting x = 2 into equation (3) gives: 2 3 + y=4 2 5 3 y = 4−1=3 5 5 y = (3) = 5 3

and since

then then

(1) (2)

Before equations (1) and (2) can be simultaneously solved, the fractions need to be removed and the equations rearranged. Multiplying equation (1) by 15 gives: x−1 y+2 2 15 + 15 = 15 3 5 15

1 1 = x 2 1 1 b= = y 5 a=

i.e.

5(x − 1) + 3(y + 2) = 2 5x − 5 + 3y + 6 = 2 5x + 3y = 2 + 5 − 6

Hence the solution is a = 12 , b = 15 , which may be checked in the original equations. Problem 10. Solve 1 4 = x+y 27 1 4 = 2x − y 33

Solve

x−1 y+2 2 + = 3 5 15 1−x 5+y 5 + = 6 2 6

43x + 0 = 86 86 =2 x= 43

1 =x a 1 =y b

7 3 =1 4 4

Hence x = 5, y = 1 34 is the required solution, which may be checked in the original equations.

Subtracting equation (5) from equation (7) gives:

Since

y=

and

Multiplying equation (6) by 6 gives: 48x + 6y = 126

4y = 27 − 20 = 7

from which

(1)

Hence

5x + 3y = 1

(3)

Multiplying equation (2) by 6 gives: 5+y 5 1−x +6 =6 6 6 2 6 i.e. (1 − x) + 3(5 + y) = 5 1 − x + 15 + 3y = 5 −x + 3y = 5 − 1 − 15

(2) Hence

−x + 3y = −11

(4)

Simultaneous equations

Thus the initial problem containing fractions can be expressed as: 5x + 3y = 1

(3)

−x + 3y = −11

(4)

Subtracting equation (4) from equation (3) gives: 6x + 0 = 12 12 x= =2 6 Substituting x = 2 into equation (3) gives: 5(2) + 3y = 1 10 + 3y = 1 3y = 1 − 10 = −9 −9 = −3 y= 3 Checking, substituting x = 2, y = −3 in equation (4) gives:

7.

65

20 5 = x + y 27 16 4 = 2x − y 33

8. If 5x −

4 5 xy + 1 3 = 1 and x + = find the value of y y 2 y

9.5 Practical problems involving simultaneous equations There are a number of situations in engineering and science where the solution of simultaneous equations is required. Some are demonstrated in the following worked problems. Problem 12. The law connecting friction F and load L for an experiment is of the form F = aL + b, where a and b are constants. When F = 5.6, L = 8.0 and when F = 4.4, L = 2.0. Find the values of a and b and the value of F when L = 6.5

LHS = −2 + 3( − 3) = −2 − 9 = −11 = RHS Hence the solution is x = 2, y = −3, which may be checked in the original equations.

Substituting F = 5.6, L = 8.0 into F = aL + b gives: 5.6 = 8.0a + b

(1)

Substituting F = 4.4, L = 2.0 into F = aL + b gives: 4.4 = 2.0a + b

Now try the following exercise Exercise 35 Further more difficult problems on simultaneous equations (Answers on page 274) In problems 1 to 7, solve the simultaneous equations and verify the results 3 2 + = 14 x y 5 3 − = −2 x y 3 1 + =5 3. 2p 5q 1.

35 5 1 = − 2 p 2p 5.

6.

4 3 − = 18 a b 2 5 + = −4 a b 5 3 4. + = 1.1 x y 2.

3 7 − = −1.1 x y

c+1 d +2 − +1=0 4 3 1 − c 3 − d 13 + + =0 5 4 20 3r + 2 2s − 1 11 − = 5 4 5 3 + 2r 5 − s 15 + = 4 3 4

(2)

Subtracting equation (2) from equation (1) gives: 1.2 = 6.0a 1 1.2 = a= 6.0 5 Substituting a = 15 into equation (1) gives: 5.6 = 8.0 15 + b 5.6 = 1.6 + b 5.6 − 1.6 = b b=4

i.e.

Checking, substituting a = 15 and b = 4 in equation (2), gives: RHS = 2.0 15 + 4 = 0.4 + 4 = 4.4 = LHS Hence a =

1 5

and b = 4

When L = 6.5, F = aL + b = 15 (6.5) + 4 = 1.3 + 4, i.e. F = 5.30 Problem 13. The equation of a straight line, of gradient m and intercept on the y-axis c, is y = mx + c. If a straight line passes through the point where x = 1 and y = −2, and also through the point where x = 3 12 and y = 10 12 , find the values of the gradient and the y-axis intercept.

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Basic Engineering Mathematics

Substituting x = 1 and y = −2 into y = mx + c gives:

Rearranging gives:

−2 = m + c

−8I1 + 10I2 = −26

(1)

Substituting x = 3 12 and y = 10 12 into y = mx + c gives:

Multiplying equation (3) by 5 gives:

10 12 = 3 12 m + c

47.5I1 − 40I2 = 135

(2)

m=

12 12 2 12

(5)

Multiplying equation (4) by 4 gives:

Subtracting equation (1) from equation (2) gives: 12 12 = 2 12 m from which,

(4)

−32I1 + 40I2 = −104

=5

(6)

Adding equations (5) and (6) gives:

Substituting m = 5 into equation (1) gives:

15.5I1 + 0 = 31 31 I2 = =2 15.5

−2 = 5 + c c = −2 − 5 = −7 Checking, substituting m = 5 and c = −7 in equation (2), gives: RHS = 3 12 (5) + (−7) = 17 12 − 7 = 10 12 = LHS

Substituting I1 = 2 into equation (3) gives: 9.5(2) − 8I1 = 27 19 − 8I2 = 27 19 − 27 = 8I2

Hence the gradient, m = 5 and the y-axis intercept, c = −7

−8 = 8I2 Problem 14. When Kirchhoff’s laws are applied to the electrical circuit shown in Figure 9.1 the currents I1 and I2 are connected by the equations: 27 = 1.5I1 + 8(I1 − I2 )

(2)

Solve the equations to find the values of currents I1 and I2 I2 (I1 ⫺ I2)

27V

Hence the solution is I1 = 2 and I2 = −1 (which may be checked in the original equations.)

(1)

−26 = 2I2 − 8(I1 − I2 )

I1

I2 = −1

26V

Substituting s = 42, t = 2 into s = ut + 12 at 2 gives:

8⍀ 1.5⍀

Problem 15. The distance s metres from a fixed point of a vehicle travelling in a straight line with constant acceleration, a m/s2 , is given by s = ut + 12 at 2 , where u is the initial velocity in m/s and t the time in seconds. Determine the initial velocity and the acceleration given that s = 42 m when t = 2 s and s = 144 m when t = 4 s. Find also the distance travelled after 3 s.

42 = 2u + 12 a(2)2

2⍀

i.e.

42 = 2u + 2a

(1)

Substituting s = 144, t = 4 into s = ut + 12 at 2 gives: Fig. 9.1 144 = 4u + 12 a(4)2 i.e. 144 = 4u + 8a

Removing the brackets from equation (1) gives: 27 = 1.5I1 + 8I1 − 8I2

Multiplying equation (1) by 2 gives: 84 = 4u + 4a

Rearranging gives: 9.5I1 − 8I2 = 27 Removing the brackets from equation (2) gives: −26 = 2I2 − 8I1 + 8I2

(2)

(3)

Subtracting equation (3) from equation (2) gives: 60 = 0 + 4a 60 = 15 a= 4

(3)

Simultaneous equations

Substituting a = 15 into equation (1) gives:

Problem 17. The resistance R of a length of wire at t ◦ C is given by R = R0 (1 + αt), where R0 is the resistance at 0◦ C and α is the temperature coefficient of resistance in/◦ C. Find the values of α and R0 if R = 30 at 50◦ C and R = 35 at 100◦ C.

42 = 2u + 2(15) 42 − 30 = 2u 12 u= =6 2 Substituting a = 15, u = 6 in equation (2) gives:

Substituting R = 30, t = 50 into R = R0 (1 + αt) gives: 30 = R0 (1 + 50α)

RHS = 4(6) + 8(15) = 24 + 120 = 144 = LHS Hence the initial velocity, u = 6 m/s and the acceleration, a = 15 m/s2 Distance travelled after 3 s is given by s = ut + 12 at 2 where t = 3, u = 6 and a = 15 Hence

s = (6)(3) +

1 (15)(3)2 2

67

= 18 + 67 12

(1)

Substituting R = 35, t = 100 into R = R0 (1 + αt) gives: 35 = R0 (1 + 100α)

(2)

Although these equations may be solved by the conventional substitution method, an easier way is to eliminate R0 by division. Thus, dividing equation (1) by equation (2) gives:

i.e. distance travelled after 3 s = 85 12 m

30 R0 (1 + 50α) 1 + 50α = = 35 R0 (1 + 100α) 1 + 100α

Problem 16. A craftsman and 4 labourers together earn £865 per week, whilst 4 craftsmen and 9 labourers earn £2340 basic per week. Determine the basic weekly wage of a craftsman and a labourer.

‘Cross-multiplying’ gives: 30(1 + 100α) = 35(1 + 50α) 30 + 3000α = 35 + 1750α 3000α − 1750α = 35 − 30

Let C represent the wage of a craftsman and L that of a labourer. Thus C + 4L = 865

(1)

4C + 9L = 2340

(2)

Multiplying equation (1) by 4 gives: 4C + 16L = 3460

(3)

Subtracting equation (2) from equation (3) gives: 7L = 1120 1120 L= = 160 7 Substituting L = 138 into equation (1) gives: C + 4(160) = 865 C + 640 = 865 C = 865 − 640 = 225 Checking, substituting C = 225 and L = 160 into equation (2), gives: LHS = 4(225) + 9(160) = 900 + 1440 = 2340 = RHS Thus the basic weekly wage of a craftsman is £225 and that of a labourer is £160

1250α = 5 1 5 = or 0.004 1250 250 1 Substituting α = into equation (1) gives: 250 1 30 = R0 1 + (50) 250 30 = R0 (1.2) 30 R0 = = 25 1.2

i.e. α =

Checking, substituting α = gives:

1 and R0 = 25 in equation (2) 250

1 RHS = 25 1 + (100) 250 = 25(1.4) = 35 = LHS Thus the solution is α = 0.004/◦ C and R0 = 25 Problem 18. The molar heat capacity of a solid compound is given by the equation c = a + bT , where a and b are constants. When c = 52, T = 100 and when c = 172, T = 400. Determine the values of a and b. When c = 52, T = 100, hence 52 = a + 100b

(1)

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Basic Engineering Mathematics

When c = 172, T = 400, hence 172 = a + 400b

(2)

Equation (2) – equation (1) gives: 120 = 300b 120 = 0.4 from which, b = 300 Substituting b = 0.4 in equation (1) gives: 52 = a + 100(0.4) a = 52 − 40 = 12 Hence a = 12

and

b = 0.4

Now try the following exercise Exercise 36 Further practical problems involving simultaneous equations (Answers on page 274) 1. In a system of pulleys, the effort P required to raise a load W is given by P = aW + b, where a and b are constants. If W = 40 when P = 12 and W = 90 when P = 22, find the values of a and b. 2. Applying Kirchhoff’s laws to an electrical circuit produces the following equations: 5 = 0.2I1 + 2(I1 − I2 ) 12 = 3I2 + 0.4I2 − 2(I1 − I2 ) Determine the values of currents I1 and I2

3. Velocity v is given by the formula v = u + at. If v = 20 when t = 2 and v = 40 when t = 7 find the values of u and a. Hence find the velocity when t = 3.5 4. Three new cars and 4 new vans supplied to a dealer together cost £97 700 and 5 new cars and 2 new vans of the same models cost £1 03 100. Find the cost of a car and a van. 5. y = mx + c is the equation of a straight line of slope m and y-axis intercept c. If the line passes through the point where x = 2 and y = 2, and also through the point where x = 5 and y = 12 , find the slope and y-axis intercept of the straight line. 6. The resistance R ohms of copper wire at t ◦ C is given by R = R0 (1 + αt), where R0 is the resistance at 0◦ C and α is the temperature coefficient of resistance. If R = 25.44 at 30◦ C and R = 32.17 at 100◦ C, find α and R0 7. The molar heat capacity of a solid compound is given by the equation c = a + bT . When c = 52, T = 100 and when c = 172, T = 400. Find the values of a and b 8. In an engineering process two variables p and q are related by: q = ap + b/p, where a and b are constants. Evaluate a and b if q = 13 when p = 2 and q = 22 when p=5

10 Quadratic equations

10.1 Introduction to quadratic equations As stated in Chapter 7, an equation is a statement that two quantities are equal and to ‘solve an equation’ means ‘to find the value of the unknown’. The value of the unknown is called the root of the equation. A quadratic equation is one in which the highest power of the unknown quantity is 2. For example, x2 − 3x + 1 = 0 is a quadratic equation. There are four methods of solving quadratic equations.

Problem 1. Solve the equations (a) x2 + 2x − 8 = 0 (b) 3x2 − 11x − 4 = 0 by factorization.

(a) x2 + 2x − 8 = 0. The factors of x2 are x and x. These are placed in brackets thus: (x )(x ) The factors of −8 are +8 and −1, or −8 and +1, or +4 and −2, or −4 and +2. The only combination to give a middle term of +2x is +4 and −2, i.e. x2 + 2x − 8 = (x + 4)(x − 2)

These are: (i) by factorization (where possible) (ii) by ‘completing the square’ (iii) by using the ‘quadratic formula’ or (iv) graphically (see Chapter 13).

10.2 Solution of quadratic equations by factorization

(Note that the product of the two inner terms added to the product of the two outer terms must equal the middle term, +2x in this case.) The quadratic equation x2 + 2x − 8 = 0 thus becomes (x + 4)(x − 2) = 0. Since the only way that this can be true is for either the first or the second, or both factors to be zero, then either (x + 4) = 0

i.e.

x = −4

(x − 2) = 0

i.e.

x=2

or Multiplying out (2x + 1)(x − 3) gives 2x2 − 6x + x − 3, i.e. 2x2 − 5x − 3. The reverse process of moving from 2x2 − 5x − 3 to (2x + 1)(x − 3) is called factorizing. If the quadratic expression can be factorized this provides the simplest method of solving a quadratic equation. For example, if 2x2 − 5x − 3 = 0, then, by factorizing: (2x + 1)(x − 3) = 0 Hence either (2x + 1) = 0

i.e.

x = − 12

(x − 3) = 0

i.e.

x=3

or

The technique of factorizing is often one of ‘trial and error’.

Hence the roots of x + 2x − 8 = 0 are x = −4 and 2 2

(b) 3x2 − 11x − 4 = 0 The factors of 3x2 are 3x and x. These are placed in brackets thus: (3x )(x ) The factors of −4 are −4 and +1, or +4 and −1, or −2 and 2. Remembering that the product of the two inner terms added to the product of the two outer terms must equal −11x, the only combination to give this is +1 and −4, i.e. 3x2 − 11x − 4 = (3x + 1)(x − 4)

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Basic Engineering Mathematics

The quadratic equation 3x2 − 11x − 4 = 0 thus becomes (3x + 1)(x − 4) = 0 Hence, either (3x + 1) = 0

i.e.

x = − 13

(x − 4) = 0

i.e.

x=4

or

and both solutions may be checked in the original equation. Problem 2. Determine the roots of (a) x − 6x + 9 = 0, and (b) 4x2 − 25 = 0, by factorization. 2

Problem 4. The roots of a quadratic equation are −2. Determine the equation.

Hence if α =

1 and β = −2, then 3 1 (x − ( − 2)) = 0 x− 3 1 (x + 2) = 0 x− 3 1 x2 − x + 2x − 3 5 x2 + x − 3

(b) 4x2 − 25 = 0 (the left-hand side is the difference of two squares, (2x)2 and (5)2 ). Thus (2x + 5)(2x − 5) = 0 i.e.

x = − 52

(2x − 5) = 0

i.e.

x = 52

or

Problem 3. Solve the following quadratic equations by factorizing: (a) 4x2 + 8x + 3 = 0 (b) 15x2 + 2x − 8 = 0. (a) 4x2 + 8x + 3 = 0. The factors of 4x2 are 4x and x or 2x and 2x. The factors of 3 are 3 and 1, or −3 and −1. Remembering that the product of the inner terms added to the product of the two outer terms must equal +8x, the only combination that is true (by trial and error) is (4x2 + 8x + 3) = (2x + 3)(2x + 1)

Thus 2x = −3, from which or

2x = −1, from which

x = − 32 x = − 12

2 =0 3 2 =0 3

3x2 + 5x − 2 = 0

Hence

Problem 5. Find the equations in x whose roots are (a) 5 and −5 (b) 1.2 and −0.4. (a) If 5 and −5 are the roots of a quadratic equation then (x − 5)(x + 5) = 0 i.e.

x2 − 5x + 5x − 25 = 0

i.e.

x 2 − 25 = 0

(b) If 1.2 and −0.4 are the roots of a quadratic equation then (x − 1.2)(x + 0.4) = 0

Hence (2x + 3)(2x + 1) = 0 from which, either (2x + 3) = 0 or (2x + 1) = 0

and

If the roots of a quadratic equation are α and β then (x − α)(x − β) = 0

(a) x2 − 6x + 9 = 0. Hence (x − 3)(x − 3) = 0, i.e. (x − 3)2 = 0 (the left-hand side is known as a perfect square). Hence x = 3 is the only root of the equation x2 − 6x + 9 = 0.

Hence either (2x + 5) = 0

1 3

i.e.

x − 1.2x + 0.4x − 0.48 = 0

i.e.

x 2 − 0.8x − 0.48 = 0

2

Now try the following exercise

which may be checked in the original equation. (b) 15x2 + 2x − 8 = 0. The factors of 15x2 are 15x and x or 5x and 3x. The factors of −8 are −4 and +2, or 4 and −2, or −8 and +1, or 8 and −1. By trial and error the only combination that works is 15x2 + 2x − 8 = (5x + 4)(3x − 2)

Exercise 37

Further problems on solving quadratic equations by factorization (Answers on page 274)

In Problems 1 to 12, solve the given equations by factorization. 1. x2 + 4x − 32 = 0

2. x2 − 16 = 0

Hence (5x + 4)(3x − 2) = 0 from which

3. (x + 2)2 = 16

4. 2x2 − x − 3 = 0

either 5x + 4 = 0

5. 6x2 − 5x + 1 = 0

6. 10x2 + 3x − 4 = 0

7. x2 − 4x + 4 = 0

8. 21x2 − 25x = 4

or

3x − 2 = 0

Hence

x = − 45

or x =

2 3

which may be checked in the original equation.

9. 8x2 + 13x − 6 = 0 11. 6x2 − 5x − 4 = 0

10. 5x2 + 13x − 6 = 0 12. 8x2 + 2x − 15 = 0

Quadratic equations

In Problems 13 to 18, determine the quadratic equations in x whose roots are 13. 3 and 1 16.

2 12

and

− 12

14. 2 and −5

15. −1 and −4

17. 6 and −6

18. 2.4 and −0.7

10.3 Solution of quadratic equations by ‘completing the square’ An expression such as x2 or (x + 2)2 or (x − 3)2 is called a perfect square. √ If x2 = 3 then x = ± 3 √ √ If (x + 2)2 = 5 then x + 2 = ± 5 and x = −2 ± 5 √ √ If (x − 3)2 = 8 then x − 3 = ± 8 and x = 3 ± 8 Hence if a quadratic equation can be rearranged so that one side of the equation is a perfect square and the other side of the equation is a number, then the solution of the equation is readily obtained by taking the square roots of each side as in the above examples. The process of rearranging one side of a quadratic equation into a perfect square before solving is called ‘completing the square’. (x + a)2 = x2 + 2ax + a2 Thus in order to make the quadratic expression x2 + 2ax into a perfect square it is necessary to add (half the coefficient of x)2 2 2a i.e. or a2 2 For example, x2 + 3x becomes a perfect square by adding 2 3 , i.e. 2 2 3 3 2 = x+ x2 + 3x + 2 2

3. Rearrange the equations so that the x2 and x terms are on one side of the equals sign and the constant is on the other side. Hence 3 5 x2 + x = 2 2 4. Add to both sides of the equation (half the coefficient of x)2 . In 5 this case the coefficient of x is . Half the coefficient squared 2 2 5 is therefore . Thus 4 5 x2 + x + 2

Problem 6. square’.

Solve 2x + 5x = 3 by ‘completing the

The procedure is as follows: 1. Rearrange the equation so that all terms are on the same side of the equals sign (and the coefficient of the x2 term is positive). Hence 2x2 + 5x − 3 = 0 2. Make the coefficient of the x2 term unity. In this case this is achieved by dividing throughout by 2. Hence

i.e.

2x2 5x 3 + − =0 2 2 2 3 5 x2 + x − = 0 2 2

2 2 5 5 3 = + 4 2 4

The LHS is now a perfect square, i.e. x+

5 4

2 =

3 + 2

2 5 4

5. Evaluate the RHS. Thus 24 + 25 49 3 25 5 2 = = = + x+ 4 2 16 16 16 6. Taking the square root of both sides of the equation (remembering that the square root of a number gives a ± answer). Thus

5 x+ 4

2

x+

i.e.

=

49 16

5 7 =± 4 4

7. Solve the simple equation. Thus 5 7 x=− ± 4 4

The method is demonstrated in the following worked problems. 2

71

i.e.

5 7 2 1 x=− + = = 4 4 4 2

and

12 5 7 x = − − = − = −3 4 4 4

Hence x = 12 or −3 are the roots of the equation 2x2 + 5x = 3 Problem 7. Solve 2x2 + 9x + 8 = 0, correct to 3 significant figures, by ‘completing the square’.

Making the coefficient of x2 unity gives: 9 x2 + x + 4 = 0 2

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Basic Engineering Mathematics

9 and rearranging gives: x2 + x = −4 2 Adding to both sides (half the coefficient of x)2 gives: 2 2 9 9 9 = −4 x2 + x + 2 4 4

Now try the following exercise Exercise 38

Further problems on solving quadratic equations by ‘completing the square’ (Answers on page 274)

In Problems 1 to 6, solve the given equations by completing the square, each correct to 3 decimal places.

The LHS is now a perfect square, thus 17 81 9 2 −4= = x+ 4 16 16

1. x2 + 4x + 1 = 0

2. 2x2 + 5x − 4 = 0

3. 3x2 − x − 5 = 0

4. 5x2 − 8x + 2 = 0

Taking the square root of both sides gives:

5. 4x − 11x + 3 = 0

6. 2x2 + 5x = 2

9 x+ = 4

2

17 = ±1.031 16

10.4 Solution of quadratic equations by formula

9 x = − ± 1.031 4

Hence

i.e. x = −1.22 or −3.28, correct to 3 significant figures. Problem 8. By ‘completing the square’, solve the quadratic equation 4.6y2 + 3.5y − 1.75 = 0, correct to 3 decimal places.

Let the general form of a quadratic equation be given by: ax2 + bx + c = 0

where a, b and c are constants.

Dividing ax2 + bx + c = 0 by a gives: c b x2 + x + = 0 a a

4.6y2 + 3.5y − 1.75 = 0

Rearranging gives:

Making the coefficient of y2 unity gives:

c b x2 + x = − a a

1.75 3.5 y− =0 y2 + 4.6 4.6

Adding to each side of the equation the square of half the coefficient of the term in x to make the LHS a perfect square gives:

and rearranging gives: y2 +

1.75 3.5 y= 4.6 4.6 2

Adding to both sides (half the coefficient of y) gives: y2 +

3.5 y+ 4.6

3.5 9.2

2 =

1.75 + 4.6

3.5 9.2

2

The LHS is now a perfect square, thus y+

3.5 9.2

2 = 0.5251654

Taking the square root of both sides gives: 3.5 √ = 0.5251654 = ±0.7246830 y+ 9.2 3.5 ± 0.7246830 9.2

Hence

y=−

i.e.

y = 0.344

or −1.105

b x2 + x + a

b 2a

2 =

b 2a

2 −

c a

Rearranging gives: x+

b a

2 =

b2 − 4ac b2 c = − 4a2 a 4a2

Taking the square root of both sides gives: √ b b2 − 4ac ± b2 − 4ac x+ = = 2a 4a2 2a √ b2 − 4ac b Hence x=− ± 2a 2a √ −b ± b2 − 4ac i.e. the quadratic formula is x = 2a (This method of solution is ‘completing the square’ – as shown in Section 10.3.)

Quadratic equations

Summarizing:

Problem 11.

if ax2 + bx + c = 0

then

√ −b ± b2 −4ac x= 2a

This is known as the quadratic formula. Problem 9. Solve (a) x2 + 2x − 8 = 0 and (b) 3x2 − 11x − 4 = 0 by using the quadratic formula. (a) Comparing x2 + 2x − 8 = 0 with ax2 + bx + c = 0 gives a = 1, b = 2 and c = −8 Substituting these values into the quadratic formula √ −b ± b2 − 4ac x= gives: 2a √ −2 ± 22 − 4(1)(−8) −2 ± 4 + 32 x= = 2(1) 2 √ −2 ± 36 −2 ± 6 = = 2 2 −2 − 6 −2 + 6 or = 2 2 −8 4 = 2 or = −4 (as in Problem 1(a)). 2 2 2 (b) Comparing 3x − 11x − 4 = 0 with ax2 + bx + c = 0 gives a = 3, b = −11 and c = −4. Hence −(−11) ± (−11)2 − 4(3)(−4) x= 2(3) √ √ +11 ± 121 + 48 11 ± 169 = = 6 6 11 + 13 11 − 13 11 ± 13 = or = 6 6 6

73

Use the quadratic formula to solve

3 x+2 + = 7 correct to 4 significant figures. 4 x−1 Multiplying throughout by 4(x − 1) gives: 3 (x + 2) + 4(x − 1) = 4(x − 1)(7) 4(x − 1) 4 (x − 1) i.e. (x − 1)(x + 2) + (4)(3) = 28(x − 1) x2 + x − 2 + 12 = 28x − 28 x2 − 27x + 38 = 0

Hence Using the quadratic formula: x= =

−(−27) ± 27 ±

√

577

2

(−27)2 − 4(1)(38) 2

=

27 ± 24.0208 2

27 + 24.0208 = 25.5104 2 27 − 24.0208 = 1.4896 or x= 2 Hence x = 25.51 or 1.490, correct to 4 significant figures.

Hence

x=

Hence x =

Hence x =

−2 1 24 = 4 or = − (as in Problem 1(b)). 6 6 3

Problem 10. Solve 4x2 + 7x + 2 = 0 giving the roots correct to 2 decimal places. Comparing 4x2 + 7x + 2 = 0 with ax2 + bx + c gives a = 4, b = 7 and c = 2. Hence

√ −7 ± [(7)2 − 4(4)(2)] −7 ± 17 x= = 2(4) 8 −7 ± 4.123 −7 + 4.123 −7 − 4.123 = = or 8 8 8

Hence x = −0.36 or −1.39, correct to 2 decimal places.

Now try the following exercise Exercise 39

Further problems on solving quadratic equations by formula (Answers on page 274)

In Problems 1 to 6 solve the given equations by using the quadratic formula, correct to 3 decimal places. 1. 2x2 + 5x − 4 = 0

2. 5.76x2 + 2.86x − 1.35 = 0

3. 2x2 − 7x + 4 = 0

4. 4x + 5 =

5. (2x + 1) =

5 x−3

6.

3 x

x+1 =x−3 x−1

10.5 Practical problems involving quadratic equations There are many practical problems where a quadratic equation has first to be obtained, from given information, before it is solved. Problem 12. The area of a rectangle is 23.6 cm2 and its width is 3.10 cm shorter than its length. Determine the dimensions of the rectangle, correct to 3 significant figures.

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Basic Engineering Mathematics

Let the length of the rectangle be x cm. Then the width is (x − 3.10) cm. Area = length × width = x(x − 3.10) = 23.6 i.e. x2 − 3.10x − 23.6 = 0 Using the quadratic formula, −(−3.10) ± (−3.10)2 − 4(1)(−23.6) x= 2(1) √ 3.10 ± 9.61 + 94.4 3.10 ± 10.20 = = 2 2 −7.10 13.30 or = 2 2 Hence x = 6.65 cm or −3.55 cm. The latter solution is neglected since length cannot be negative.

Problem 14. The height s metres of a mass projected vertically upwards at time t seconds is s = ut − 12 gt 2 . Determine how long the mass will take after being projected to reach a height of 16 m (a) on the ascent and (b) on the descent, when u = 30 m/s and g = 9.81 m/s2 . When height s = 16 m, 16 = 30 t − 12 (9.81)t 2 i.e.

4.905t 2 − 30t + 16 = 0

Using the quadratic formula: −(−30) ± (−30)2 − 4(4.905)(16) t= 2(4.905) =

Thus length x = 6.65 cm and width = x − 3.10 = 6.65 − 3.10 = 3.55 cm. Hence the dimensions of the rectangle are 6.65 cm by 3.55 cm. (Check: Area = 6.65 × 3.55 = 23.6 cm2 , correct to 3 significant figures.)

√ 30 ± 24.21 30 ± 586.1 = = 5.53 9.81 9.81

0.59

Hence the mass will reach a height of 16 m after 0.59 s on the ascent and after 5.53 s on the descent. Problem 15. A shed is 4.0 m long and 2.0 m wide. A concrete path of constant width is laid all the way around the shed. If the area of the path is 9.50 m2 calculate its width to the nearest centimetre.

Problem 13. Calculate the diameter of a solid cylinder which has a height of 82.0 cm and a total surface area of 2.0 m2 .

Figure 10.1 shows a plan view of the shed with its surrounding path of width t metres.

Total surface area of a cylinder = curved surface area + 2 circular ends

t

= 2πrh + 2π r (where r = radius and h = height) 2

t

Since the total surface area = 2.0 m2 and the height h = 82 cm or 0.82 m, then

2.0 m

2.0 = 2π r(0.82) + 2πr 2 i.e. 2πr 2 + 2πr(0.82) − 2.0 = 0 Dividing throughout by 2π gives:

or

r 2 + 0.82r −

4.0 m

1 =0 π

Using the quadratic formula: −0.82 ± (0.82)2 − 4(1) − π1 r= 2(1) √ −0.82 ± 1.3948 −0.82 ± 1.9456 = = 2 2

SHED

Fig. 10.1 Area of path = 2(2.0 × t) + 2t(4.0 + 2t) i.e.

= 0.2874 or −1.1074

Hence the diameter of the cylinder

= 2 × 0.2874 = 0.5748 m

57.5 cm correct to 3 significant figures

or

9.50 = 4.0t + 8.0t + 4t 2

4t 2 + 12.0t − 9.50 = 0 −(12.0) ± (12.0)2 − 4(4)(−9.50) Hence t = 2(4) √ −12.0 ± 296.0 −12.0 ± 17.20465 = = 8 8 Hence t = 0.6506 m or −3.65058 m or

Thus the radius r of the cylinder is 0.2874 m (the negative solution being neglected).

(4.0 ⫹ 2t)

Quadratic equations

Neglecting the negative result which is meaningless, the width of the path, t = 0.651 m or 65 cm, correct to the nearest centimetre.

5. The acid dissociation constant Ka of ethanoic acid is 1.8 × 10−5 mol dm−3 for a particular solution. Using x2 determine x, the the Ostwald dilution law Ka = v(l − x) degree of ionization, given that v = 10 dm3 .

Problem 16. If the total surface area of a solid cone is 486.2 cm2 and its slant height is 15.3 cm, determine its base diameter.

6. A rectangular building is 15 m long by 11 m wide. A concrete path of constant width is laid all the way around the building. If the area of the path is 60.0 m2 , calculate its width correct to the nearest millimetre.

From Chapter 24, page 180, the total surface area A of a solid cone is given by: A = πrl + πr 2 where l is the slant height and r the base radius.

7. The total surface area of a closed cylindrical container is 20.0 m3 . Calculate the radius of the cylinder if its height is 2.80 m2 .

A = 482.2 and l = 15.3, then 482.2 = πr(15.3) + πr 2

If i.e.

π r 2 + 15.3π r − 482.2 = 0

8. The bending moment M at a point in a beam is given 3x(20 − x) where x metres is the distance from by M = 2 the point of support. Determine the value of x when the bending moment is 50 Nm.

482.2 r 2 + 15.3r − =0 π

or

Using the quadratic formula, r= =

−482.2 (15.3)2 − 4 π

−15.3 ±

9. A tennis court measures 24 m by 11 m. In the layout of a number of courts an area of ground must be allowed for at the ends and at the sides of each court. If a border of constant width is allowed around each court and the total area of the court and its border is 950 m2 , find the width of the borders.

2 −15.3 ±

√

848.0461

2

=

−15.3 ± 29.12123 2

10. Two resistors, when connected in series, have a total resistance of 40 ohms. When connected in parallel their total resistance is 8.4 ohms. If one of the resistors has a resistance Rx ohms:

Hence radius r = 6.9106 cm (or −22.21 cm, which is meaningless, and is thus ignored). Thus the diameter of the base

75

= 2r = 2(6.9106)

(a) show that R2x − 40Rx + 336 = 0 and

= 13.82 cm

Now try the following exercise Exercise 40 Further practical problems involving quadratic equations (Answers on page 274) 1. The angle of a rotating shaft turns through in t seconds is given by θ = ωt + 12 αt 2 . Determine the time taken to complete 4 radians if ω is 3.0 rad/s and α is 0.60 rad/s2 .

(b) calculate the resistance of each

10.6 The solution of linear and quadratic equations simultaneously Sometimes a linear equation and a quadratic equation need to be solved simultaneously. An algebraic method of solution is shown in Problem 17; a graphical solution is shown in Chapter 13, page 99.

2. The power P developed in an electrical circuit is given by P = 10I − 8I 2 , where I is the current in amperes. Determine the current necessary to produce a power of 2.5 watts in the circuit.

Problem 17. Determine the values of x and y which simultaneously satisfy the equations: y = 5x − 4 − 2x2 and y = 6x − 7

3. The area of a triangle is 47.6 cm2 and its perpendicular height is 4.3 cm more than its base length. Determine the length of the base correct to 3 significant figures.

For a simultaneous solution the values of y must be equal, hence the RHS of each equation is equated. Thus

4. The sag l metres in a cable stretched between two 12 + x. supports, distance x m apart is given by: l = x Determine the distance between supports when the sag is 20 m.

5x − 4 − 2x2 = 6x − 7 Rearranging gives: i.e. or

5x − 4 − 2x2 − 6x + 7 = 0 −x + 3 − 2x2 = 0 2x2 + x − 3 = 0

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Basic Engineering Mathematics

Factorizing gives:

(2x + 3)(x − 1) = 0

3 i.e. x=− or x = 1 2 In the equation y = 6x − 7, −3 3 − 7 = −16 when x = − , y = 6 2 2 and

when x = 1, y = 6 − 7 = −1

[Checking the result in y = 5x − 4 − 2x2 : 3 3 2 3 −4−2 − when x = − , y = 5 − 2 2 2 9 15 = − − 4 − = −16 2 2 as above; and when x = 1, y = 5 − 4 − 2 = −1 as above] 3 Hence the simultaneous solutions occur when x = − , y = −16 2 and when x = 1, y = −1

Now try the following exercise Exercise 41

Further problems on solving linear and quadratic equations simultaneously (Answers on page 275)

In Problems 1 to 3 determine the solutions of the simultaneous equations. 1. y = x2 + x + 1 y=4−x 2. y = 15x2 + 21x − 11 y = 2x − 1 3. 2x2 + y = 4 + 5x x+y=4

11 Inequalities

11.1 Introduction to inequalities

11.2 Simple inequalities

An inequality is any expression involving one of the symbols <, >, ≤ or ≥

The solution of some simple inequalities, using only the rules given in Section 11.1, is demonstrated in the following worked problems.

p < q means p is less than q p > q means p is greater than q

Problem 1. Solve the following inequalities: p (a) 3 + x > 7 (b) 3t < 6 (c) z − 2 ≥ 5 (d) ≤ 2 3

p ≤ q means p is less than or equal to q p ≥ q means p is greater than or equal to q Some simple rules (i) When a quantity is added or subtracted to both sides of an inequality, the inequality still remains.

3 + x − 3 > 7 − 3, i.e. x > 4

For example, if p < 3 then

p + 2 < 3 + 2 (adding 2 to both sides)

and

p − 2 < 3 − 2 (subtracting 2 from both sides)

(ii) When multiplying or dividing both sides of an inequality by a positive quantity, say 5, the inequality remains the same. For example, if p > 4

then

5p > 20

and

4 p > 5 5

(iii) When multiplying or dividing both sides of an inequality by a negative quantity, say −3, the inequality is reversed. For example, if p > 1

(a) Subtracting 3 from both sides of the inequality: 3 + x > 7 gives:

then −3p < −3

and

p 1 < −3 −3

(Note > has changed to < in each example.) To solve an inequality means finding all the values of the variable for which the inequality is true. Knowledge of simple equations (Chapter 7) and quadratic equations (Chapter 10) are needed in this chapter.

Hence, all values of x greater than 4 satisfy the inequality. (b) Dividing both sides of the inequality: 3t < 6 by 3 gives: 6 3t < , i.e. t < 2 3 3 Hence, all values of t less than 2 satisfy the inequality. (c) Adding 2 to both sides of the inequality z − 2 ≥ 5 gives: z − 2 + 2 ≥ 5 + 2, i.e. z ≥ 7 Hence, all values of z greater than or equal to 7 satisfy the inequality. p (d) Multiplying both sides of the inequality ≤ 2 by 3 gives: 3 p (3) ≤ (3)2, i.e. p ≤ 6 3 Hence, all values of p less than or equal to 6 satisfy the inequality. Problem 2.

Solve the inequality: 4x + 1 > x + 5

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Basic Engineering Mathematics

Subtracting 1 from both sides of the inequality: 4x + 1 > x + 5 gives: 4x > x + 4 Subtracting x from both sides of the inequality: 4x > x + 4 gives: 3x > 4 Dividing both sides of the inequality: 3x > 4 by 3 gives: x>

4 3

Hence all values of x greater than

4 3

satisfy the inequality:

4x + 1 > x + 5

Problem 3.

For example, | 4 | = 4 and | −4 | = 4 (the modulus of a number is never negative), The inequality: | t | < 1 means that all numbers whose actual size, regardless of sign, is less than 1, i.e. any value between −1 and +1. Thus | t | < 1 means −1 < t < 1. Similarly, | x | > 3 means all numbers whose actual size, regardless of sign, is greater than 3, i.e. any value greater than 3 and any value less than −3. Thus | x | > 3 means x > 3 and x < −3. Inequalities involving a modulus are demonstrated in the following worked problems. Problem 4.

Solve the inequality: 3 − 4t ≤ 8 + t

Subtracting 3 from both sides of the inequality: 3 − 4t ≤ 8 + t gives: −4t ≤ 5 + t Subtracting t from both sides of the inequality: −4t ≤ 5 + t gives: −5t ≤ 5 Dividing both sides of the inequality: −5t ≤ 5 by −5 gives:

Solve the following inequality: | 3x + 1 | < 4

Since | 3x + 1 | < 4

then

−4 < 3x + 1 < 4

Now −4 < 3x + 1 becomes −5 < 3x, 5 i.e. − < x and 3x + 1 < 4 becomes 3x < 3, i.e. x < 1 3 5 Hence, these two results together become − < x < 1 and mean 3 that the inequality | 3x + 1 | < 4 is satisfied for any value of x 5 greater than − but less than 1. 3 Problem 5.

Solve the inequality: |1 + 2t| ≤ 5

t ≥ −1 (remembering to reverse the inequality) Hence, all values of t greater than or equal to −1 satisfy the inequality.

Since |1 + 2t| ≤ 5

Exercise 42 Further problems on simple inequalities (Answers on page 275) Solve the following inequalities: 1. (a) 3t > 6 x 2. (a) > 1.5 2 3. (a) 4t − 1 ≤ 3 7 − 2k ≤1 4 5. (a) 5 − 2y ≤ 9 + y 4. (a)

(b) 2x < 10

1 + 2t ≤ 5 becomes 2t ≤ 4, i.e. t ≤ 2

Hence, these two results together become: −3 ≤ t ≤ 2 Problem 6.

Solve the inequality: |3z − 4| > 2

|3z − 4| > 2 means 3z − 4 > 2 i.e. 3z > 6

(b) x + 2 ≥ 5

−5 ≤ 1 + 2t ≤ 5

Now −5 ≤ 1 + 2t becomes −6 ≤ 2t, i.e. −3 ≤ t and

Now try the following exercise

then

and

and

3z − 4 < −2,

3z < 2,

i.e. the inequality: |3z − 4| > 2 is satisfied when z > 2 and z < 23

(b) 5 − x ≥ −1 (b) 3z + 2 > z + 3 (b) 1 − 6x ≤ 5 + 2x

11.3 Inequalities involving a modulus The modulus of a number is the size of the number, regardless of sign. Vertical lines enclosing the number denote a modulus.

Now try the following exercise Exercise 43

Further problems on inequalities involving a modulus (Answers on page 275)

Solve the following inequalities: 1. | t + 1 | < 4

2. | y + 3 | ≤ 2

4. |3t − 5| > 4

5. |1 − k| ≥ 3

3. |2x − 1| < 4

Inequalities

11.4 Inequalities involving quotients p p > 0 then must be a positive value. q q p For to be positive, either p is positive and q is positive or p is q negative and q is negative. If

i.e.

+ =+ +

and

+ =− −

and

Since

2x + 3 − (x + 2) ≤0 x+2

For

x+1 to be negative or zero, x+2 or

− =− +

Solve the inequality:

t +1 >0 3t − 6

i.e.

either

This reasoning is used when solving inequalities involving quotients, as demonstrated in the following worked problems.

Problem 7.

2x + 3 x + 2 ≤ 0, − x+2 x+2

− =+ −

p p must be a negative value. If < 0 then q q p For to be negative, either p is positive and q is negative or p q is negative and q is positive. i.e.

i.e.

then

t+1 >0 3t − 6

t +1 must be positive. 3t − 6

t +1 to be positive, either (i) t + 1 > 0 and 3t − 6 > 0 or 3t − 6 (ii) t + 1 < 0 and 3t − 6 < 0

x+1 ≤0 x+2

or

(i) x + 1 ≤ 0

and

x+2>0

(ii) x + 1 ≥ 0 and

x+2<0

(i) If x + 1 ≤ 0 then x ≤ −1 and if x + 2 > 0 then x > −2 (Note that > is used for the denominator, not ≥; a zero denominator gives a value for the fraction which is impossible to evaluate.) x+1 Hence, the inequality ≤ 0 is true when x is greater than x+2 −2 and less than or equal to −1, which may be written as −2 < x ≤ −1 (ii) If x + 1 ≥ 0 then x ≥ −1 and if x + 2 < 0 then x < −2 It is not possible to satisfy both x ≥ −1 and x < −2 thus no values of x satisfies (ii). Summarizing,

2x + 3 ≤1 x+2

when

Exercise 44

(i) If t + 1 > 0 then t > −1 and if 3t − 6 > 0 then 3t > 6 and t >2

Solve the following inequalities:

(ii) If t + 1 < 0 then t < −1 and if 3t − 6 < 0 then 3t < 6 and t <2 Both of the inequalities t < −1 and t < 2 are only true when t < −1, t +1 is positive when t < −1 i.e. the fraction 3t − 6 t +1 > 0 when t > 2 or t < −1 Summarizing, 3t − 6

Problem 8.

Solve the inequality:

2x + 3 ≤1 x+2

2x + 3 ≤ 1 then x+2

2x + 3 −1≤0 x+2

Further problems on inequalities involving quotients (Answers on page 275)

1.

x+4 ≥0 6 − 2x

2.

2t + 4 >1 t −5

3.

3z − 4 ≤2 z+5

4.

2−x ≥4 x+3

11.5 Inequalities involving square functions The following two general rules apply when inequalities involve square functions: √ √ (i) if x2 > k then x > k or x < − k (1) √ √ (2) (ii) if x2 < k then − k < x < k These rules are demonstrated in the following worked problems. Problem 9.

Since

−2 < x ≤ −1

Now try the following exercise

For

Both of the inequalities t > −1 and t > 2 are only true when t > 2, t +1 is positive when t > 2 i.e. the fraction 3t − 6

79

Solve the inequality: t 2 > 9

Since t 2 > 9 then t 2 − 9 > 0, i.e. (t + 3)(t − 3) > 0 by factorizing

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Basic Engineering Mathematics

For (t + 3)(t − 3) to be positive, either

(i) (t + 3) > 0

and

(t − 3) > 0

or

(ii) (t + 3) < 0

and

(t − 3) < 0

This demonstrates the general rule: √ √ if x 2 < k then − k < x < k Problem 13.

(i) If (t + 3) > 0 then t > −3 and if (t − 3) > 0 then t > 3

(2)

Solve the inequality: x2 < 4

Both of these are true only when t > 3 From the general rule stated above in equation (2): √ √ if x2 < 4 then − 4 < x < 4

(ii) If (t + 3) < 0 then t < −3 and if (t − 3) < 0 then t < 3 Both of these are true only when t < −3 2

Summarizing, t > 9

when

i.e. the inequality: x2 < 4 is satisfied when: −2 < x < 2

t > 3 or t < −3

This demonstrates the general rule: √ √ if x 2 > k then x > k or x < − k

Problem 14.

Solve the inequality: ( y − 3)2 ≤ 16

(1) From equation (2),

Problem 10.

Solve the inequality: x2 > 4

From the general rule stated above in equation (1): √ √ if x2 > 4 then x > 4 or x < − 4

√ √ − 16 ≤ (y − 3) ≤ 16 −4 ≤ (y − 3) ≤ 4

i.e.

3 − 4 ≤ y ≤ 4 + 3,

from which,

−1 ≤ y ≤ 7

i.e.

i.e. the inequality: x > 4 is satisfied when x > 2 or x < −2 2

Now try the following exercise Problem 11.

Solve the inequality: (2z + 1)2 > 9 Exercise 45

From equation (1), if (2z + 1)2 > 9 then √ √ 2z + 1 > 9 or 2z + 1 < − 9 i.e.

2z + 1 > 3

i.e.

2z > 2

i.e.

z>1

or 2z + 1 < −3 or 2z < −4, or

Solve the following inequalities: 1. z 2 > 16

2. z 2 < 16

3. 2x2 ≥ 6

4. 3k 2 − 2 ≤ 10

5. (t − 1)2 ≤ 36

6. (t − 1)2 ≥ 36

7. 7 − 3y2 ≤ −5 8. (4k + 5)2 > 9

z < −2

Problem 12. Solve the inequality: t 2 < 9 Since t 2 < 9 then t 2 − 9 < 0, i.e. (t + 3)(t − 3) < 0 by factorizing. For (t + 3)(t − 3) to be negative, (i) (t + 3) > 0

and

(t − 3) < 0

or (ii) (t + 3) < 0

and

(t − 3) > 0

either

Further problems on inequalities involving square functions (Answers on page 275)

(i) If (t + 3) > 0 then t > −3 and if (t − 3) < 0 then t < 3 Hence (i) is satisfied when t > −3 and t < 3 which may be written as: −3 < t < 3 (ii) If (t + 3) < 0 then t < −3 and if (t − 3) > 0 then t > 3 It is not possible to satisfy both t < −3 and t > 3, thus no values of t satisfies (ii). Summarizing, t 2 < 9 when −3 < t < 3 which means that all values of t between −3 and +3 will satisfy the inequality.

11.6 Quadratic inequalities Inequalities involving quadratic expressions are solved using either factorization or ‘completing the square’. For example, x2 − 2x − 3 is factorized as (x + 1)(x − 3) and

6x2 + 7x − 5 is factorized as (2x − 1)(3x + 5)

If a quadratic expression does not factorize, then the technique of ‘completing the square’ is used. In general, the procedure for x2 + bx + c is: 2 b b 2 2 +c− x + bx + c ≡ x + 2 2 For example, x2 + 4x − 7 does not factorize; completing the square gives: x2 + 4x − 7 ≡ (x + 2)2 − 7 − 22 ≡ (x + 2)2 − 11 Similarly,

x2 − 6x − 5 ≡ (x − 3)2 − 5 − 32 ≡ (x − 3)2 − 14

Inequalities

Solving quadratic inequalities is demonstrated in the following worked problems. Problem 15.

Solve the inequality: x2 + 2x − 3 > 0

Since x2 + 2x − 3 > 0 then (x − 1)(x + 3) > 0 by factorizing. For the product (x − 1)(x + 3) to be positive, either

(i) (x − 1) > 0 and

(x + 3) > 0

or

(ii) (x − 1) < 0 and

(x + 3) < 0

(i) Since (x − 1) > 0 then x > 1 and since (x + 3) > 0 then x > −3

81

x2 + 6x + 3 does not factorize; completing the square gives: x2 + 6x + 3 ≡ (x + 3)2 + 3 − 32 ≡ (x + 3)2 − 6 The inequality thus becomes: (x + 3)2 − 6 < 0 or (x + 3)2 < 6 √ √ From equation (2), − 6 < (x + 3) < 6

√ √ 6−3 from which, − 6 − 3 < x < Hence, x2 + 6x + 3 < 0 is satisfied when −5.45 < x < −0.55 correct to 2 decimal places.

Problem 18.

Solve the inequality: y2 − 8y − 10 ≥ 0

Both of these inequalities are satisfied only when x > 1 (ii) Since (x − 1) < 0 then x < 1 and since (x + 3) < 0 then x < −3 Both of these inequalities are satisfied only when x < −3 Summarizing, x2 + 2x − 3 > 0 is satisfied when either x > 1 or x < −3 Problem 16.

Solve the inequality: t 2 − 2t − 8 < 0

Since t 2 − 2t − 8 < 0 then (t − 4)(t + 2) < 0 by factorizing. For the product (t − 4)(t + 2) to be negative, either

(i) (t − 4) > 0

and

(t + 2) < 0

or

(ii) (t − 4) < 0

and

(t + 2) > 0

(i) Since (t − 4) > 0 then t > 4 and since (t + 2) < 0 then t < −2 It is not possible to satisfy both t > 4 and t < −2 , thus no values of t satisfies the inequality (i) (ii) Since (t − 4) < 0 then t < 4 and since (t + 2) > 0 then t > −2 Hence, (ii) is satisfied when −2 < t < 4 Summarizing, t 2 − 2t − 8 < 0 is satisfied when −2 < t < 4 Problem 17.

Solve the inequality: x2 + 6x + 3 < 0

y2 − 8y − 10 does not factorize; completing the square gives: y2 − 8y − 10 ≡ ( y − 4)2 − 10 − 42 ≡ ( y − 4)2 − 26 The inequality thus becomes: ( y − 4)2 − 26 ≥ 0 or ( y − 4)2 ≥ 26 √ √ From equation (1), ( y − 4) ≥ 26 or (y − 4) ≤ − 26 √ √ from which, y ≥ 4 + 26 or y ≤ 4 − 26 Hence, y2 − 8y − 10 ≥ 0 is satisfied when y ≥ 9.10 y ≤ −1.10 correct to 2 decimal places.

Now try the following exercise Exercise 46

Further problems on quadratic inequalities (Answers on page 275)

Solve the following inequalities: 1. x2 − x − 6 > 0

2. t 2 + 2t − 8 ≤ 0

3. 2x2 + 3x − 2 < 0

4. y2 − y − 20 ≥ 0

5. z 2 + 4z + 4 ≤ 4

6. x2 + 6x + 6 ≤ 0

7. t 2 − 4t − 7 ≥ 0

8. k 2 + k − 3 ≥ 0

or

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Basic Engineering Mathematics

Assignment 5

3. Solve the following equations by factorization: (a) x2 − 9 = 0

(b) 2x2 − 5x − 3 = 0

(6)

This assignment covers the material contained in Chapters 9 to 11. The marks for each question are shown in brackets at the end of each question.

4. Determine the quadratic equation in x whose roots are 1 and −3 (4)

1. Solve the following pairs of simultaneous equations:

6. Solve the equation 4x2 − 9x + 3 = 0 correct to 3 decimal places. (5)

(a) 7x − 3y = 23 2x + 4y = −8 b (b) 3a − 8 + = 0 8 a 21 b+ = 2 4 2p + 1 1 − 4q 5 = − 2 2 5 1 − 3p 2q − 3 32 + + =0 (20) 7 5 35 2. In an engineering process two variables x and y are b related by the equation y = ax + where a and b are x constants. Evaluate a and b if y = 15 when x = 1 and y = 13 when x = 3 (5) (c)

5. Solve the equation 3x2 − x − 4 = 0 by completing the square. (6)

7. The current i flowing through an electronic device is given by: i = 0.005 v 2 + 0.014 v where v is the voltage. Calculate the values of v when i = 3 × 10−3 (6) 8. Solve the following inequalities: (a) 2 − 5x ≤ 9 + 2x (d) (3t + 2)2 > 16

x−1 >0 3x + 5 (e) 2x2 − x − 3 < 0 (18) (b) |3 + 2t| ≤ 6 (c)

12 Straight line graphs

12.1 Introduction to graphs

12.2 The straight line graph

A graph is a pictorial representation of information showing how one quantity varies with another related quantity. The most common method of showing a relationship between two sets of data is to use Cartesian or rectangular axes as shown in Fig. 12.1.

Let a relationship between two variables x and y be y = 3x + 2. When x = 0, y = 3(0) + 2 = 2. When x = 1, y = 3(1) + 2 = 5. When x = 2, y = 3(2) + 2 = 8, and so on. Thus co-ordinates (0, 2), (1, 5) and (2, 8) have been produced from the equation by selecting arbitrary values of x, and are shown plotted in Fig. 12.2. When the points are joined together a straight-line graph results.

y 4 B (⫺4, 3) 3 2 Origin ⫺4

⫺3 ⫺2

⫺1

C (⫺3, ⫺2)

y

1 0

A (3, 2)

Abscissa

Ordinate

A 1

2

8 3

4

x

6

⫺1

4

⫺2

2

⫺3

⫺1

0

y ⫽ 3x ⫹ 2

1

2

x

⫺4

Fig. 12.1 The points on a graph are called co-ordinates. Point A in Fig. 12.1 has the co-ordinates (3, 2), i.e. 3 units in the x direction and 2 units in the y direction. Similarly, point B has coordinates (−4, 3) and C has co-ordinates (−3, −2). The origin has co-ordinates (0, 0). The horizontal distance of a point from the vertical axis is called the abscissa and the vertical distance from the horizontal axis is called the ordinate.

Fig. 12.2 The gradient or slope of a straight line is the ratio of the change in the value of y to the change in the value of x between any two points on the line. If, as x increases, (→), y also increases (↑), then the gradient is positive. In Fig. 12.3(a), the gradient of AC =

change in y CB 7−3 4 = = = =2 change in x BA 3−1 2

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Basic Engineering Mathematics

y ⫽ ⫺3x ⫹ 2

y ⫽ 2x ⫹ 1

8 7 6 5 4 3 2 1 ⫺1 0

11

F

y

(iv) The scales need not start at zero, particularly when starting at zero produces an accumulation of points within a small area of the graph paper.

y 10 8

C

(v) The co-ordinates, or points, should be clearly marked. This may be done either by a cross, or a dot and circle, or just by a dot (see Fig. 12.1).

6 4 A 1

B 2

3

E 4

D

⫺4 ⫺3 ⫺2 ⫺1 0

x

(a)

(vi) A statement should be made next to each axis explaining the numbers represented with their appropriate units.

2 x

(vii) Sufficient numbers should be written next to each axis without cramping.

(b) y 3

y⫽3

2

Problem 1. Plot the graph y = 4x + 3 in the range x = −3 to x = +4. From the graph, find (a) the value of y when x = 2.2, and (b) the value of x when y = −3.

1 0

1

2

3 x

(c)

Fig. 12.3

Whenever an equation is given and a graph is required, a table giving corresponding values of the variable is necessary. The table is achieved as follows: When x = −3, y = 4x + 3 = 4(−3) + 3 = −12 + 3 = −9. When x = −2, y = 4(−2) + 3 = −8 + 3 = −5, and so on.

If as x increases (→), y decreases (↓), then the gradient is negative. In Fig. 12.3(b), the gradient of DF =

change in y FE 11 − 2 = = change in x ED −3 − 0 9 = = −3 −3

Figure 12.3(c) shows a straight line graph y = 3. Since the straight line is horizontal the gradient is zero. The value of y when x = 0 is called the y-axis intercept. In Fig. 12.3(a) the y-axis intercept is 1 and in Fig. 12.3(b) is 2. If the equation of a graph is of the form y = mx + c, where m and c are constants, the graph will always be a straight line, m representing the gradient and c the y-axis intercept. Thus y = 5x + 2 represents a straight line of gradient 5 and y-axis intercept 2. Similarly, y = −3x − 4 represents a straight line of gradient −3 and y-axis intercept −4

Such a table is shown below: x y

−3 −9

−2 −5

1 7

2 11

3 15

4 19

(a) when x = 2.2, y = 11.8, and (b) when y = −3, x = −1.5 y 20 15 11.8 10 5 ⫺1.5 ⫺3

(ii) Choose scales such that the graph occupies as much space as possible on the graph paper being used. (iii) Choose scales so that interpolation is made as easy as possible. Usually scales such as 1 cm = 1 unit, or 1 cm = 2 units, or 1 cm = 10 units are used. Awkward scales such as 1 cm = 3 units or 1 cm = 7 units should not be used.

0 3

The co-ordinates (−3, −9), (−2, −5), (−1, −1), and so on, are plotted and joined together to produce the straight line shown in Fig. 12.4. (Note that the scales used on the x and y axes do not have to be the same) From the graph:

Summary of general rules to be applied when drawing graphs (i) Give the graph a title clearly explaining what is being illustrated.

−1 −1

⫺2

⫺1

0 ⫺5 ⫺10

Fig. 12.4

1 ⫺3

2 2.2

3

4 x

Straight line graphs

Problem 2. Plot the following graphs on the same axes between the range x = −4 to x = +4, and determine the gradient of each. (a) y = x

(b) y = x + 2

(c) y = x + 5

(d) y = x − 3

A table of co-ordinates is produced for each graph. (a) y = x x y

−4 −4

−3 −3

−2 −2

−1 −1

0 0

1 1

2 2

3 3

4 4

(d) y = x − 3

x y

−4 −2

−3 −1

−2 0

−1 1

0 2

1 3

2 4

3 5

4 6

(c) y = x + 5 x y

−4 1

−3 −6

−2 −1 −5 −4

0 −3

1 −2

2 −1

3 0

4 1

The co-ordinates are plotted and joined for each graph. The results are shown in Fig. 12.5. Each of the straight lines produced are parallel to each other, i.e. the slope or gradient is the same for each. To find the gradient of any straight line, say, y = x − 3 a horizontal and vertical component needs to be constructed. In Fig. 12.5, AB is constructed vertically at x = 4 and BC constructed horizontally at y = −3. The gradient of AC AB 1−(−3) 4 = = =1 BC 4−0 4

i.e. the gradient of the straight line y = x − 3 is 1. The actual positioning of AB and BC is unimportant for the gradient is also given by DE −1−(−2) 1 = = =1 EF 2−1 1 The slope or gradient of each of the straight lines in Fig. 12.5 is thus 1 since they are all parallel to each other.

−3 2

−2 3

−1 4

0 5

1 6

2 7

3 8

y 9

y⫽

8

x⫹

5

x⫹

2

7 6

y⫽

5 4

y⫽

3

x

⫺4 ⫺3 ⫺2 ⫺1

1 ⫺1 ⫺2 ⫺3 ⫺4 ⫺5 ⫺6

F C

(a) y = 3x

(b) y = 3x + 7

(c) y = −4x + 4

(d) y = −4x − 5

x y

4

E B

−3 −9

−2 −6

−1 −3

0 0

1 3

2 6

3 9

2 13

3 16

(b) y = 3x + 7

3A

y⫽ 2 3 D

Problem 3. Plot the following graphs on the same axes between the values x = −3 to x = +3 and determine the gradient and y-axis intercept of each.

A table of co-ordinates is drawn up for each equation.

x⫺

1

4 9

(a) y = 3x

2

x

x y

−3 −2

−2 1

−1 4

0 7

1 10

−2 12

−1 8

0 4

1 0

2 −4

3 −8

1 −9

2 −13

3 −17

(c) y = −4x + 4 x y

−3 16

(d) y = −4x − 5

⫺7

Fig. 12.5

−4 −7

x y

= (b) y = x + 2

85

x y

−3 7

−2 3

−1 −1

0 −5

86

Basic Engineering Mathematics

Each of the graphs is plotted as shown in Fig. 12.6, and each is a straight line. y = 3x and y = 3x + 7 are parallel to each other and thus have the same gradient. The gradient of AC is given by CB 16 − 7 9 = = =3 BA 3−0 3

y⫽ ⫺ 4 ⫹ 4x

y⫽

y⫽

8

⫺

7 C

3x B

A

4x 4

5 ⫺ ⫺2

⫹

12

y⫽ ⫺3

3x

(iii) c represents the y-axis intercept. Thus, given an equation such as y = 3x + 7, it may be deduced ‘on sight’ that its gradient is +3 and its y-axis intercept is +7, as shown in Fig. 12.6. Similarly, if y = −4x − 5, then the gradient is −4 and the y-axis intercept is −5, as shown in Fig. 12.6. When plotting a graph of the form y = mx + c, only two co-ordinates need be determined. When the co-ordinates are plotted a straight line is drawn between the two points. Normally, three co-ordinates are determined, the third one acting as a check.

y 16

(i) a graph of y against x produces a straight line, (ii) m represents the slope or gradient of the line, and

⫺1

0

1

2

3

x

⫺4 F ⫺8 ⫺12 ⫺16 E

D

Fig. 12.6 Hence the gradient of both y = 3x and y = 3x + 7 is 3. y = −4x + 4 and y = −4x − 5 are parallel to each other and thus have the same gradient. The gradient of DF is given by

Problem 4. The following equations represent straight lines. Determine, without plotting graphs, the gradient and y-axis intercept for each. (a) y = 3

(b) y = 2x

(c) y = 5x − 1

(d) 2x + 3y = 3

(a) y = 3 (which is of the form y = 0x + 3) represents a horizontal straight line intercepting the y-axis at 3. Since the line is horizontal its gradient is zero. (b) y = 2x is of the form y = mx + c, where c is zero. Hence gradient = 2 and y-axis intercept = 0 (i.e. the origin). (c) y = 5x − 1 is of the form y = mx + c. Hence gradient = 5 and y-axis intercept = −1 (d) 2x + 3y = 3 is not in the form y = mx + c as it stands. Transposing to make y the subject gives 3y = 3 − 2x, i.e.

FE −5 − ( − 17) 12 = = = −4 ED 0−3 −3 Hence the gradient of both y = −4x + 4 and y = −4x − 5 is −4. The y-axis intercept means the value of y where the straight line cuts the y-axis. From Fig. 12.6, y = 3x cuts the y-axis at y = 0

and

3 2x 3 − 2x = − 3 3 3 2x y = − +1 3 y=

i.e.

which is of the form y = mx + c. Hence gradient = − 23 and y-axis intercept = +1

y = 3x + 7 cuts the y-axis at y = +7

Problem 5. Without plotting graphs, determine the gradient and y-axis intercept values of the following equations:

y = −4x + 4 cuts the y-axis at y = +4

(a) y = 7x − 3

y = −4x − 5 cuts the y-axis at y = −5

(c) y − 2 = 4x + 9

Some general conclusions can be drawn from the graphs shown in Figs 12.4, 12.5 and 12.6. When an equation is of the form y = mx + c, where m and c are constants, then

(b) 3y = −6x + 2 y x 1 (d) = − 3 3 5

(e) 2x + 9y + 1 = 0 (a) y = 7x − 3 is of the form y = mx + c, hence gradient, m = 7 and y-axis intercept, c = −3.

Straight line graphs

(b) A straight line passes through (−2, −3) and (−1, 3), hence x1 = −2, y1 = −3, x2 = −1 and y2 = 3,

(b) Rearranging 3y = −6x + 2 gives 2 6x + 3 3 2 y = −2x + 3 y=−

i.e.

y2 − y 1 3 − (−3) = x2 − x 1 −1 − (−2) 3+3 6 = = =6 −1 + 2 1

hence gradient, m =

which is of the form y = mx + c. Hence gradient m = −2 and y-axis intercept, c = 23 (c) Rearranging y − 2 = 4x + 9 gives y = 4x + 11, hence gradient = 4 and y-axis intercept = 11 y x 1 (d) Rearranging = − gives 3 2 5 x 1 3 3 y=3 − = x− 2 5 2 5 Hence gradient = 32 and y-axis intercept = − 35 (e) Rearranging 2x + 9y + 1 = 0 gives

i.e.

9y = −2x − 1, 2 1 y = − x− 9 9

Problem 7. Plot the graph 3x + y + 1 = 0 and 2y − 5 = x on the same axes and find their point of intersection.

Rearranging 3x + y + 1 = 0 gives y = −3x − 1 Rearranging 2y − 5 = x gives 2y = x + 5 and y = 12 x + 2 12 Since both equations are of the form y = mx + c both are straight lines. Knowing an equation is a straight line means that only two co-ordinates need to be plotted and a straight line drawn through them. A third co-ordinate is usually determined to act as a check. A table of values is produced for each equation as shown below.

Hence gradient = − 29 and y-axis intercept = − 19 Problem 6. Determine the gradient of the straight line graph passing through the co-ordinates (a) (−2, 5) and (3, 4), and (b) (−2, −3) and (−1, 3) A straight line graph passing through co-ordinates (x1 , y1 ) and (x2 , y2 ) has a gradient given by: y2 − y 1 x2 − x 1

1

0

−1

−4

−1

2

2

0

−3

3 12

2 12

1

x −3x − 1

m=

x 1 x + 2 12 2

The graphs are plotted as shown in Fig. 12.8 y

(see Fig. 12.7)

4 y ⫽ ⫺3x ⫺1

y

3

(x2, y2)

y2 (x1, y1)

5 y ⫽1 x ⫹ 2 2

2 (y2 ⫺ y1)

1

y1 (x2 ⫺ x1) 0

x1

⫺4

⫺3

⫺2

⫺1

0

1

2

3

4

⫺1

x2 x

⫺2

Fig. 12.7

⫺3

(a) A straight line passes through (−2, 5) and (3, 4), hence x1 = −2, y1 = 5, x2 = 3 and y2 = 4, y2 − y1 x2 − x 1 1 4−5 =− = 3 − ( − 2) 5

hence gradient m =

87

⫺4

Fig. 12.8 The two straight lines are seen to intersect at (−1, 2)

x

88

Basic Engineering Mathematics

Now try the following exercise Exercise 47 Further problems on straight line graphs (Answers on page 275) 1. Corresponding values obtained experimentally for two quantities are: x y

−2.0 −0.5 −13.0 −5.5

0 −3.0

1.0 2.5 2.0 9.5

3.0 12.0

5.0 22.0

Use a horizontal scale for x of 1 cm = 12 unit and a vertical scale for y of 1 cm = 2 units and draw a graph of x against y. Label the graph and each of its axes. By interpolation, find from the graph the value of y when x is 3.5 2. The equation of a line is 4y = 2x + 5. A table of corresponding values is produced and is shown below. Complete the table and plot a graph of y against x. Find the gradient of the graph. x −4 −3 y

−2 −1 0

−0.25

3.25

3. Determine the gradient and intercept on the y-axis for each of the following equations: (b) y = −x

(c) y = −3x − 4

(d) y = 4

4. Find the gradient and intercept on the y-axis for each of the following equations: (a) 2y − 1 = 4x

(b) 6x − 2y = 5

x 4 Determine the gradient and y-axis intercept for each of the equations in Problems 5 and 6 and sketch the graphs. (c) 3(2y − 1) =

5. (a) y = 6x − 3 (c) y = 3x

(a) y − 4 = 2x 1 (c) x = (y + 5) 2 1 (e) 2x = (7 − y) 2

(b) 4x = −(y + 1) 1 3 (d) 1 + y = x 2 2

9. Draw a graph of y − 3x + 5 = 0 over a range of x = −3 to x = 4. Hence determine (a) the value of y when x = 1.3 and (b) the value of x when y = −9.2 10. Draw on the same axes the graphs of y = 3x − 5 and 3y + 2x = 7. Find the co-ordinates of the point of intersection. Check the result obtained by solving the two simultaneous equations algebraically. 11. Plot the graphs y = 2x + 3 and 2y = 15 − 2x on the same axes and determine their point of intersection.

1 2 3 4

1.25

(a) y = 4x − 2

8. State which of the following equations will produce graphs which are parallel to one another:

12.3 Practical problems involving straight line graphs When a set of co-ordinate values are given or are obtained experimentally and it is believed that they follow a law of the form y = mx + c, then if a straight line can be drawn reasonably close to most of the co-ordinate values when plotted, this verifies that a law of the form y = mx + c exists. From the graph, constants m (i.e. gradient) and c (i.e. y-axis intercept) can be determined. This technique is called determination of law (see Chapter 16). Problem 8. The temperature in degrees Celsius and the corresponding values in degrees Fahrenheit are shown in the table below. Construct rectangular axes, choose a suitable scale and plot a graph of degrees Celsius (on the horizontal axis) against degrees Fahrenheit (on the vertical scale).

(b) y = −2x + 4 (d) y = 7

6. (a) 2y + 1 = 4x x (c) 3(2y − 4) = 3

(b) 2x + 3y + 5 = 0 y 7 (d) 5x − − = 0 2 3

7. Determine the gradient of the straight line graphs passing through the co-ordinates: (a) (2, 7) and (−3, 4) (b) (−4, −1) and (−5, 3) 3 1 1 5 ,− (c) and − , 4 4 2 8

◦ ◦

C F

10 50

20 68

40 104

60 140

80 176

100 212

From the graph find (a) the temperature in degrees Fahrenheit at 55◦ C, (b) the temperature in degrees Celsius at 167◦ F, (c) the Fahrenheit temperature at 0◦ C, and (d) the Celsius temperature at 230◦ F. The co-ordinates (10, 50), (20, 68), (40, 104), and so on are plotted as shown in Fig. 12.9. When the co-ordinates are joined, a straight line is produced. Since a straight line results there is a linear relationship between degrees Celsius and degrees Fahrenheit.

Straight line graphs

y

89

Plot a graph of volume (vertical) against temperature (horizontal) and from it find (a) the temperature when the volume is 28.6 m3 , and (b) the volume when the temperature is 67◦ C.

240 230

Degrees Fahrenheit (°F)

200 167 160 131 120

E

If a graph is plotted with both the scales starting at zero then the result is as shown in Fig. 12.10. All of the points lie in the top right-hand corner of the graph, making interpolation difficult. A more accurate graph is obtained if the temperature axis starts at 55◦ C and the volume axis starts at 24.5 m3 . The axes corresponding to these values is shown by the broken lines in Fig. 12.10 and are called false axes, since the origin is not now at zero. A magnified version of this relevant part of the graph is shown in Fig. 12.11. From the graph:

F

D B

80

40 32 A 0

20

40

y

G

55 60

75 80

30

100 110 120 x

Degrees Celsius (°C)

25

Fig. 12.9

◦

Volume (m3)

20

(a) To find the Fahrenheit temperature at 55◦ C a vertical line AB is constructed from the horizontal axis to meet the straight line at B. The point where the horizontal line BD meets the vertical axis indicates the equivalent Fahrenheit temperature.

15 10

◦

Hence 55 C is equivalent to 131 F. 5

This process of finding an equivalent value in between the given information in the above table is called interpolation. (b) To find the Celsius temperature at 167◦ F, a horizontal line EF is constructed as shown in Fig. 12.9. The point where the vertical line FG cuts the horizontal axis indicates the equivalent Celsius temperature.

0

20

40 60 80 Temperature (°C)

100 x

Fig. 12.10

Hence 167◦ F is equivalent to 75◦ C. (c) If the graph is assumed to be linear even outside of the given data, then the graph may be extended at both ends (shown by broken line in Fig. 12.9). From Fig. 12.9, 0◦ C corresponds to 32◦ F.

(a) when the volume is 28.6 m3 , the equivalent temperature is 82.5◦ C, and (b) when the temperature is 67◦ C, the equivalent volume is 26.1 m3

(d) 230◦ F is seen to correspond to 110◦ C. The process of finding equivalent values outside of the given range is called extrapolation.

Problem 9. In an experiment on Charles’s law, the value of the volume of gas, V m3 , was measured for various temperatures T ◦ C. Results are shown below. V m3 T ◦C

25.0 60

25.8 65

26.6 70

27.4 75

28.2 80

29.0 85

Problem 10. In an experiment demonstrating Hooke’s law, the strain in an aluminium wire was measured for various stresses. The results were: Stress N/mm2 Strain

4.9 0.00007

8.7 0.00013

15.0 0.00021

Stress N/mm2 Strain

18.4 0.00027

24.2 0.00034

27.3 0.00039

90

Basic Engineering Mathematics

y 28

(a) Young’s Modulus of Elasticity for aluminium which is given by the gradient of the graph,

24

(b) the value of the strain at a stress of 20 N/mm2 , and

20

(c) the value of the stress when the strain is 0.00020

y

Stress (N/mm2)

Plot a graph of stress (vertically) against strain (horizontally). Find:

A

16 14 12 8

29

B

C 4

28.6

0

0.00005

0.00015

28

0.00025 Strain

0.00035

x

0.000285

Volume (m3)

Fig. 12.12

27

Thus Young’s Modulus of Elasticity for aluminium is 70 000 N/mm2 Since 1 m2 = 106 mm2 , 70 000 N/mm2 is equivalent to 70 000 × 106 N/m2 , i.e. 70 × 109 N/m2 (or Pascals) From Fig. 12.12:

26.1 26

(b) the value of the strain at a stress of 20 N/mm2 is 0.000285, and (c) the value of the stress when the strain is 0.00020 is 14 N/mm2 Problem 11. The following values of resistance R ohms and corresponding voltage V volts are obtained from a test on a filament lamp.

25

55

60

65 67 70 75 Temperature (°C)

80 82.5 85 x

Fig. 12.11 The co-ordinates (0.00007, 4.9), (0.00013, 8.7), and so on, are plotted as shown in Fig. 12.12. The graph produced is the best straight line which can be drawn corresponding to these points. (With experimental results it is unlikely that all the points will lie exactly on a straight line.) The graph, and each of its axes, are labelled. Since the straight line passes through the origin, then stress is directly proportional to strain for the given range of values. (a) The gradient of the straight line AC is given by AB 28 − 7 21 = = BC 0.00040 − 0.00010 0.00030 7 21 = −4 = 3 × 10−4 10 = 7 × 104 = 70 000 N/mm2

R ohms V volts

30 16

48.5 29

73 52

107 76

128 94

Choose suitable scales and plot a graph with R representing the vertical axis and V the horizontal axis. Determine (a) the gradient of the graph, (b) the R axis intercept value, (c) the equation of the graph, (d) the value of resistance when the voltage is 60 V, and (e) the value of the voltage when the resistance is 40 ohms, (f) If the graph were to continue in the same manner, what value of resistance would be obtained at 110 V? The co-ordinates (16, 30), (29, 48.5), and so on, are shown plotted in Fig. 12.13 where the best straight line is drawn through the points. (a) The slope or gradient of the straight line AC is given by 135 − 10 125 AB = = = 1.25 BC 100 − 0 100 (Note that the vertical line AB and the horizontal line BC may be constructed anywhere along the length of the straight line.

Straight line graphs

Show that the values obey the law σ = at + b, where a and b are constants and determine approximate values for a and b. Use the law to determine the stress at 250◦ C and the temperature when the stress is 7.54 N/cm2 .

y 147 140

A

120

Resistance R ohms

91

The co-ordinates (70, 8.46), (200, 8.04), and so on, are plotted as shown in Fig. 12.14. Since the graph is a straight line then the values obey the law σ = at + b, and the gradient of the straight line is

100 85 80

a=

60

AB 8.36 − 6.76 1.60 = = = −0.0032 BC 100 − 600 −500

y 8.68

40

8.50 8.36

20

0

A

B

C

8.00 20 24

40

60

80

100 110 120 x

Voltage V volts

Fig. 12.13 However, calculations are made easier if the horizontal line BC is carefully chosen, in this case, 100).

Stress s N/cm2

10

(b) The R-axis intercept is at R = 10 ohms (by extrapolation).

7.50

7.00

(c) The equation of a straight line is y = mx + c, when y is plotted on the vertical axis and x on the horizontal axis. m represents the gradient and c the y-axis intercept. In this case, R corresponds to y, V corresponds to x, m = 1.25 and c = 10. Hence the equation of the graph is R = (1.25 V + 10).

6.76 6.50 0

B 100

C 200

300 400 500 Temperature t °C

600

From Fig. 12.13, Fig. 12.14

(d) when the voltage is 60 V, the resistance is 85 (e) when the resistance is 40 ohms, the voltage is 24V, and

Vertical axis intercept, b = 8.68

(f ) by extrapolation, when the voltage is 110 V, the resistance is 147 .

Hence the law of the graph is σ = 0.0032t + 8.68

Problem 12. Experimental tests to determine the breaking stress σ of rolled copper at various temperatures t gave the following results

2

Stress σ N/cm Temperature t ◦ C

8.46 70

8.04 200

7.78 280

When the temperature is 250◦ C, stress σ is given by σ = −0.0032(250) + 8.68 = 7.88 N/cm2 Rearranging σ = −0.0032t + 8.68 gives

i.e.

0.0032t = 8.68 − σ , 8.68 − σ t= 0.0032

Hence when the stress σ = 7.54 N/cm2 , temperature Stress σ N/cm2 Temperature t ◦ C

7.37 410

7.08 500

6.63 640

t=

8.68 − 7.54 = 356.3◦ C 0.0032

700 x

92

Basic Engineering Mathematics

Now try the following exercise Exercise 48 Further practical problems involving straight line graphs (Answers on page 275) 1. The resistance R ohms of a copper winding is measured at various temperatures t ◦ C and the results are as follows: R ohms t◦C

112 20

120 36

126 48

131 58

134 64

Plot a graph of R (vertically) against t (horizontally) and find from it (a) the temperature when the resistance is 122 and (b) the resistance when the temperature is 52◦ C. 2. The speed of a motor varies with armature voltage as shown by the following experimental results: n (rev/min) 285 517 615 750 917 1050 V volts 60 95 110 130 155 175 Plot a graph of speed (horizontally) against voltage (vertically) and draw the best straight line through the points. Find from the graph (a) the speed at a voltage of 145 V, and (b) the voltage at a speed of 400 rev/min. 3. The following table gives the force F newtons which, when applied to a lifting machine, overcomes a corresponding load of L newtons. Force F newtons 25 Load L newtons 50

47

64

120

149

187

140

210

430

550

700

Choose suitable scales and plot a graph of F (vertically) against L (horizontally). Draw the best straight line through the points. Determine from the graph (a) the gradient, (b) the F-axis intercept, (c) the equation of the graph, (d) the force applied when the load is 310 N, and (e) the load that a force of 160 N will overcome. (f) If the graph were to continue in the same manner, what value of force will be needed to overcome a 800 N load? 4. The following table gives the results of tests carried out to determine the breaking stress σ of rolled copper at various temperatures, t: Stress σ (N/cm2 ) Temperature t(◦ C)

8.51 75

8.07 220

7.80 310

Stress σ (N/cm2 ) Temperature t(◦ C)

7.47 420

7.23 500

6.78 650

Plot a graph of stress (vertically) against temperature (horizontally). Draw the best straight line through the plotted co-ordinates. Determine the slope of the graph and the vertical axis intercept. 5. The velocity v of a body after varying time intervals t was measured as follows: t (seconds) v (m/s)

2 16.9

5 19.0

8 11 15 18 21.1 23.2 26.0 28.1

Plot v vertically and t horizontally and draw a graph of velocity against time. Determine from the graph (a) the velocity after 10 s, (b) the time at 20 m/s and (c) the equation of the graph. 6. The mass m of a steel joint varies with length L as follows: mass, m (kg) 80 100 120 140 160 length, L (m) 3.00 3.74 4.48 5.23 5.97 Plot a graph of mass (vertically) against length (horizontally). Determine the equation of the graph. 7. The crushing strength of mortar varies with the percentage of water used in its preparation, as shown below. Crushing strength, F (tonnes) 1.64 1.36 1.07 0.78 0.50 0.22 % of water used, w% 6 9 12 15 18 21 Plot a graph of F (vertically) against w (horizontally). (a) Interpolate and determine the crushing strength when 10% of water is used. (b) Assuming the graph continues in the same manner extrapolate and determine the percentage of water used when the crushing strength is 0.15 tonnes. (c) What is the equation of the graph? 8. In an experiment demonstrating Hooke’s law, the strain in a copper wire was measured for various stresses. The results were: Stress (pascals) Strain

10.6 × 106 0.00011

18.2 × 106 0.00019

24.0 × 106 0.00025

Straight line graphs

Stress (pascals) Strain

30.7 × 106 0.00032

39.4 × 106 0.00041

Plot a graph of stress (vertically) against strain (horizontally). Determine (a) Young’s Modulus of Elasticity for copper, which is given by the gradient of the graph, (b) the value of strain at a stress of 21 × 106 Pa, (c) the value of stress when the strain is 0.00030 9. An experiment with a set of pulley blocks gave the following results: Effort, E (newtons) 9.0 11.0 13.6 17.4 20.8 23.6 Load, L (newtons) 15 25 38 57 74 88

93

Plot a graph of effort (vertically) against load (horizontally) and determine (a) the gradient, (b) the vertical axis intercept, (c) the law of the graph, (d) the effort when the load is 30 N and (e) the load when the effort is 19 N 10. The variation of pressure p in a vessel with temperature T is believed to follow a law of the form p = aT + b, where a and b are constants. Verify this law for the results given below and determine the approximate values of a and b. Hence determine the pressures at temperatures of 285 K and 310 K and the temperature at a pressure of 250 kPa. Pressure, p kPa 244 247 252 258 262 267 Temperature, TK 273 277 282 289 294 300

13 Graphical solution of equations

13.1 Graphical solution of simultaneous equations

y 5

Linear simultaneous equations in two unknowns may be solved graphically by:

3

(ii) noting their point of intersection.

2

The co-ordinates of the point of intersection give the required solution.

1 ⫺4

⫺3

⫺2 ⫺1

0

1

2

3

4

x

⫺1

Solve graphically the simultaneous equations

2x − y = 4

⫺2

x+y = 5

⫺3 ⫺4

Rearranging each equation into y = mx + c form gives: y = 2x − 4

(1)

y = −x + 5

(2)

Only three co-ordinates need be calculated for each graph since both are straight lines. x y = 2x − 4

y ⫽ 2x ⫺4

4

(i) plotting the two straight lines on the same axes, and

Problem 1.

y ⫽ ⫺x ⫹ 5

0 −4

1 −2

Fig. 13.1

Problem 2.

Solve graphically the equations

1.20x + y = 1.80 x − 5.0y = 8.50

2 0

Rearranging each equation into y = mx + c form gives: x y = −x + 5

0 5

1 4

2 3

Each of the graphs is plotted as shown in Fig. 13.1. The point of intersection is at (3, 2) and since this is the only point which lies simultaneously on both lines then x = 3, y = 2 is the solution of the simultaneous equations.

i.e.

y = −1.20x + 1.80

(1)

8.5 x − y= 5.0 5.0 y = 0.20x − 1.70

(2)

Three co-ordinates are calculated for each equation as shown below.

Graphical solution of equations

x y = −1.20x + 1.80

x y = 0.20x − 1.70

0 1.80

1 0.60

2 −0.60

0 −1.70

1 −1.50

2 −1.30

The two lines are plotted as shown in Fig. 13.2. The point of intersection is (2.50, −1.20). Hence the solution of the simultaneous equation is x = 2.50, y = −1.20. (It is sometimes useful initially to sketch the two straight lines to determine the region where the point of intersection is. Then, for greater accuracy, a graph having a smaller range of values can be drawn to ‘magnify’ the point of intersection). y 3 2 1 2.50 ⫺3

⫺2

⫺1

0

1

⫺1 ⫺1.20

2

3

4

x

95

a and b are constants. When F = 4 newtons, L = 6 newtons and when F = 2.4 newtons, L = 2 newtons. Determine graphically the values of a and b.

13.2 Graphical solutions of quadratic equations A general quadratic equation is of the form y = ax2 + bx + c, where a, b and c are constants and a is not equal to zero. A graph of a quadratic equation always produces a shape called a parabola. The gradient of the curve between 0 and A and between B and C in Fig. 13.3 is positive, whilst the gradient between A and B is negative. Points such as A and B are called turning points. At A the gradient is zero and, as x increases, the gradient of the curve changes from positive just before A to negative just after. Such a point is called a maximum value. At B the gradient is also zero, and, as x increases, the gradient of the curve changes from negative just before B to positive just after. Such a point is called a minimum value.

y ⫽ 0.20x ⫹ 1.70

⫺2

y

⫺3

C A

y ⫽ ⫺1.20x ⫹ 1.80

Fig. 13.2 B x

0

Now try the following exercise Fig. 13.3 Exercise 49 Further problems on the graphical solution of simultaneous equations (Answers on page 275) In Problems 1 to 5, solve the simultaneous equations graphically. 1. x + y = 2 3y − 2x = 1 3. 3x + 4y = 5 2x − 5y + 12 = 0

2. y = 5 − x x−y=2 4. 1.4x − 7.06 = 3.2y 2.1x − 6.7y = 12.87

5. 3x − 2y = 0 4x + y + 11 = 0

Quadratic graphs (i) y = ax 2 Graphs of y = x2 , y = 3x2 and y = 12 x2 are shown in Fig. 13.4.

2

y ⫽ x2

⫺1 0

Fig. 13.4

2

y ⫽ 3x 2

1 x

⫺1 0 (b)

y 2

y⫽

1 2

1

x

1

1

1

(a)

6. The friction force F newtons and load L newtons are connected by a law of the form F = aL + b, where

y

y

1 x

⫺1 0 (c)

x2

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Basic Engineering Mathematics

When y = ax2 + c:

All have minimum values at the origin (0, 0).

(a) curves are symmetrical about the y-axis,

Graphs of y = −x2 , y = −3x2 and y = − 12 x2 are shown in Fig. 13.5.

(b) the magnitude of ‘a’ affects the gradient of the curve, and (c) the constant ‘c’ is the y-axis intercept.

0 ⫺1 ⫺1

0 ⫺1 y ⫽ ⫺x 2 ⫺1 x

1

⫺2 y

⫺2 y (a)

0 x ⫺1 1 ⫺1 y ⫽ ⫺3x 2 ⫺2 y

(b)

(iii) y = ax 2 + bx + c

x 1

y ⫽⫺

1 2

x2

(c)

Whenever ‘b’ has a value other than zero the curve is displaced to the right or left of the y-axis. When b/a is positive, the curve is displaced b/2a to the left of the y-axis, as shown in Fig. 13.7(a). When b/a is negative the curve is displaced b/2a to the right of the y-axis, as shown in Fig. 13.7(b).

Fig. 13.5

y 12 10

All have maximum values at the origin (0, 0).

y

8 y ⫽ x 2 ⫹ 6x ⫹ 11 6

6

(a) curves are symmetrical about the y-axis,

4

4

(b) the magnitude of ‘a’ affects the gradient of the curve,

2

2

⫺5 ⫺4 ⫺3 ⫺2 ⫺1 0

1 x ⫺1 0

When y = ax2 :

and (c) the sign of ‘a’ determines whether it has a maximum or minimum value.

Graphs of y = x2 + 3, y = x2 − 2, y = −x2 + 2 and y = −2x2 − 1 are shown in Fig. 13.6.

y y ⫽ x2 ⫹ 3

y ⫽ x2 ⫺ 2

2 3 ⫺1

⫺1

2

3

4

x

⫺2 (b)

Fig. 13.7 Quadratic equations of the form ax2 + bx + c = 0 may be solved graphically by: (i) plotting the graph y = ax2 + bx + c, and

y

0

1

0

1

x

⫺2

x

y 0

y ⫺1

2

0

1

x

⫺1

(c)

(d)

The x values of the points of intersection give the required solutions since at these points both y = 0 and ax2 + bx + c = 0

Problem 3. 1

Solve the quadratic equation

x

4x2 + 4x − 15 = 0

⫺4

y ⫽ ⫺x 2 ⫹ 2

(ii) noting the points of intersection on the x-axis (i.e. where y = 0).

The number of solutions, or roots of a quadratic equation, depends on how many times the curve cuts the x-axis and there can be no real roots (as in Fig. 13.7(a)) or one root (as in Figs 13.4 and 13.5) or two roots (as in Fig. 13.7(b)).

(b)

(a)

Fig. 13.6

1

(a)

(ii) y = ax 2 + c

⫺1

y ⫽ x 2 ⫺5x ⫹ 4

graphically given that the solutions lie in the range x = −3 to x = 2. Determine also the co-ordinates and nature of the turning point of the curve. y ⫽ ⫺2x 2 ⫺1

Let y = 4x2 + 4x − 15. A table of values is drawn up as shown below.

Graphical solution of equations

−3

x

−2

−1

0

1

y

2

y ⫽ 4x 2

30

4x2 4x −15

36 −12 −15

y = 4x2 + 4x − 15

16 −8 −15

4 −4 −15

−7 −15

9

0 4 0 4 −15 −15 −15

16 8 −15

−7

97

25 20

9

A graph of y = 4x2 + 4x − 15 is shown in Fig. 13.8. The only points where y = 4x2 + 4x − 15 and y = 0 are the points marked A and B. This occurs at x = −2.5 and x = 1.5 and these are the solutions of the quadratic equation 4x2 + 4x − 15 = 0. (By substituting x = −2.5 and x = 1.5 into the original equation the solutions may be checked.) The curve has a turning point at (−0.5, −16) and the nature of the point is a minimum.

15 10 y ⫽ ⫺4x ⫹ 5

5 ⫺3

⫺2

⫺1

1

0

⫺2.5

3

2

x

1.5

y

Fig. 13.9

y ⫽ 4x 2 ⫹ 4x ⫺ 15

12

y 12 11.25

8 4 ⫺0.5

A ⫺3

⫺2

10

⫺1

⫺2.5

0

B 1

⫺4

y ⫽ ⫺5x 2 ⫹ 9x ⫹ 7.2

8 2

x 6

1.5

4 ⫺8 ⫺12

2 ⫺1

⫺16

0 −0.6 ⫺2

⫺6

An alternative graphical method of solving

⫺8

4x2 + 4x − 15 = 0

4x2 = −4x + 15, i.e. 4x2 + 4x − 15 = 0. This is shown in Fig. 13.9, where the roots are x = −2.5 and x = 1.5 as before. Problem 4. Solve graphically the quadratic equation −5x2 + 9x + 7.2 = 0 given that the solutions lie between x = −1 and x = 3. Determine also the co-ordinates of the turning point and state its nature.

2

3

x

2.4

⫺4

Fig. 13.8

is to rearrange the equation as 4x2 = −4x + 15 and then plot two separate graphs – in this case y = 4x2 and y = −4x + 15. Their points of intersection give the roots of equation

1 0.9

⫺10

Fig. 13.10 Let y = −5x2 + 9x + 7.2. A table of values is drawn up as shown on page 98. A graph of y = −5x2 + 9x + 7.2 is shown plotted in Fig. 13.10. The graph crosses the x-axis (i.e. where y = 0) at x = −0.6 and x = 2.4 and these are the solutions of the quadratic equation −5x2 + 9x + 7.2 = 0 The turning point is a maximum having co-ordinates (0.9, 11.25)

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Basic Engineering Mathematics

x

−1

−0.5

0

−5x2 +9x +7.2

−5 −9 7.2

−1.25 −4.5 7.2

0 0 7.2

−5 9 7.2

y = −5x2 + 9x + 7.2

−6.8

1.45

7.2

11.2

x

2

−5x2 +9x +7.2

−31.25 22.5 7.2

−45 27 7.2

5.2

−1.55

−10.8

y = −5x + 9x + 7.2

Problem 6. Plot the graph of y = −2x2 + 3x + 6 for values of x from x = −2 to x = 4. Use the graph to find the roots of the following equations: (a) −2x2 + 3x + 6 = 0 (b) −2x2 + 3x + 2 = 0 (c) −2x2 + 3x + 9 = 0 (d) −2x2 + x + 5 = 0

A table of values is drawn up as shown below.

3

−20 18 7.2 2

Problem 5. equations:

2.5

1

x

−2

−1

0

1

2

3

4

−2x2 +3x +6

−8 −6 6

−2 −3 6

0 0 6

−2 3 6

−8 6 6

−18 9 6

−32 12 6

y

−8

1

6

7

4

−3

−14

Plot a graph of y = 2x and hence solve the 2

A graph of −2x2 + 3x + 6 is shown in Fig. 13.12.

(a) 2x − 8 = 0 and (b) 2x − x − 3 = 0 2

2

y

A graph of y = 2x2 is shown in Fig. 13.11. y ⫽ ⫺2x 2 ⫹ 3x ⫹ 6

8

y y ⫽ 2x 2

6 A

y ⫽ 2x ⫹ 1

H

10 B

8

C

y⫽8

D

4

y⫽4

6 D

y⫽x

4 C

⫺2

⫺1

⫹3

2

2

⫺1.35 ⫺1.13 A ⫺2 ⫺1 ⫺0.5 0

0

⫺1.5

1

1.5

2

x

(b) Rearranging 2x2 − x − 3 = 0 gives 2x2 = x + 3 and the solution of this equation is obtained from the points of intersection of y = 2x2 and y = x + 3, i.e. at C and D in Fig. 13.11. Hence the solutions of 2x2 − x − 3 = 0 are x = −1 and x = 1.5

2

3

x

⫺2

E

F ⫺4

(a) Rearranging 2x2 − 8 = 0 gives 2x2 = 8 and the solution of this equation is obtained from the points of intersection of y = 2x2 and y = 8, i.e. at co-ordinates (−2, 8) and (2, 8), shown as A and B, respectively, in Fig. 13.11. Hence the solutions of 2x2 − 8 = 0 are x = −2 and x = +2

1

1.85 2.63 G

Fig. 13.11

B

y ⫽ ⫺3

⫺6 ⫺8

Fig. 13.12 (a) The parabola y = −2x2 + 3x + 6 and the straight line y = 0 intersect at A and B, where x = −1.13 and x = 2.63 and these are the roots of the equation −2x2 + 3x + 6 = 0

Graphical solution of equations

(b) Comparing y = −2x2 + 3x + 6

(1)

0 = −2x2 + 3x + 2

(2)

with

shows that if 4 is added to both sides of equation (2), the right-hand side of both equations will be the same. Hence 4 = −2x2 + 3x + 6. The solution of this equation is found from the points of intersection of the line y = 4 and the parabola y = −2x2 + 3x + 6, i.e. points C and D in Fig. 13.12. Hence the roots of −2x2 + 3x + 2 = 0 are x = −0.5 and x = 2

−2x2 + 3x + 6 = −3 and the solution of this equation is obtained from the points of intersection of the line y = −3 and the parabola y = −2x2 + 3x + 6x, i.e. at points E and F in Fig. 13.12. Hence the roots of −2x2 + 3x + 9 = 0 are x = −1.5 and x=3 (3)

0 = −2x2 + x + 5

with

to x = 1. Determine also the nature and co-ordinates of its turning point. 7. Solve graphically the quadratic equation 10x2 − 9x − 11.2 = 0 given that the roots lie between x = −1 and x = 2 8. Plot a graph of y = 3x2 and hence solve the equations. (a) 3x2 − 8 = 0 and (b) 3x2 − 2x − 1 = 0 9. Plot the graphs y = 2x2 and y = 3 − 4x on the same axes and find the co-ordinates of the points of intersection. Hence determine the roots of the equation 2x2 + 4x − 3 = 0

(c) −2x2 + 3x + 9 = 0 may be rearranged as

(d) Comparing y = −2x2 + 3x + 6

99

10. Plot a graph of y = 10x2 − 13x − 30 for values of x between x = −2 and x = 3. Solve the equation 10x2 − 13x − 30 = 0 and from the graph determine (a) the value of y when x is 1.3, (b) the value of x when y is 10 and (c) the roots of the equation 10x2 − 15x − 18 = 0

(4)

shows that if 2x + 1 is added to both sides of equation (4) the right-hand side of both equations will be the same. Hence equation (4) may be written as 2x + 1 = −2x2 + 3x + 6 The solution of this equation is found from the points of intersection of the line y = 2x + 1 and the parabola y = −2x2 + 3x + 6, i.e. points G and H in Fig. 13.12. Hence the roots of −2x2 + x + 5 = 0 are x = −1.35 and x = 1.85

13.3 Graphical solution of linear and quadratic equations simultaneously The solution of linear and quadratic equations simultaneously may be achieved graphically by: (i) plotting the straight line and parabola on the same axes, and (ii) noting the points of intersection. The co-ordinates of the points of intersection give the required solutions. Problem 7. Determine graphically the values of x and y which simultaneously satisfy the equations

Now try the following exercise

y = 2x2 − 3x − 4 and y = 2 − 4x Exercise 50 Further problems on solving quadratic equations graphically (Answers on page 276) 1. Sketch the following graphs and state the nature and co-ordinates of their turning points (a) y = 4x

2

(c) y = −x2 + 3

(b) y = 2x − 1 2

(d) y = − 12 x2 − 1

Solve graphically the quadratic equations in Problems 2 to 5 by plotting the curves between the given limits. Give answers correct to 1 decimal place. 2. 4x2 − x − 1 = 0; x = −1 to x = 1 3. x2 − 3x = 27; x = −5 to x = 8 4. 2x2 − 6x − 9 = 0; x = −2 to x = 5 5. 2x(5x − 2) = 39.6; x = −2 to x = 3 6. Solve the quadratic equation 2x2 + 7x + 6 = 0 graphically, given that the solutions lie in the range x = −3

y = 2x2 − 3x − 4 is a parabola and a table of values is drawn up as shown below: x

−2

−1

0

1

2

3

2x2 −3x −4

8 6 −4

2 3 −4

0 0 −4

2 −3 −4

8 −6 −4

18 −9 −4

10

1

−4

−5

−2

5

y

y = 2 − 4x is a straight line and only three co-ordinates need be calculated: x y

0 2

1 −2

2 −6

The two graphs are plotted in Fig. 13.13 and the points of intersection, shown as A and B, are at co-ordinates (−2,10) and (1 12 , −4).

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Basic Engineering Mathematics

y 10

A

8 y ⫽ 2x 2 ⫺3x ⫺4

6

and (ii) noting the points of intersection on the x-axis (i.e. where y = 0). The x-values of the points of intersection give the required solution since at these points both y = 0 and ax3 + bx2 + cx + d = 0. The number of solutions, or roots of a cubic equation depends on how many times the curve cuts the x-axis and there can be one, two or three possible roots, as shown in Fig. 13.14.

4

y

y

y

2

⫺2

⫺1

x 0

1

2

3

x

⫺2

(a)

⫺4

x

x

B

(b)

(c)

Fig. 13.14 y ⫽ 2⫺4x

Problem 8. Fig. 13.13

Solve graphically the cubic equation

4x3 − 8x2 − 15x + 9 = 0

Hence the simultaneous solutions occur when x = −2, y = 10 and when x = 1 12 , y = −4 (These solutions may be checked by substituting into each of the original equations)

Now try the following exercise Exercise 51 Further problems on solving linear and quadratic equations simultaneously (Answers on page 276) 1. Determine graphically the values of x and y which simultaneously satisfy the equations y = 2(x2 − 2x − 4) and y + 4 = 3x 2. Plot the graph of y = 4x2 − 8x − 21 for values of x from −2 to +4. Use the graph to find the roots of the following equations: (a) 4x2 − 8x − 21 = 0

(b) 4x2 − 8x − 16 = 0

(c) 4x − 6x − 18 = 0 2

13.4 Graphical solution of cubic equations A cubic equation of the form ax3 + bx2 + cx + d = 0 may be solved graphically by: (i) plotting the graph y = ax3 + bx2 + cx + d

given that the roots lie between x = −2 and x = 3. Determine also the co-ordinates of the turning points and distinguish between them.

Let y = 4x3 − 8x2 − 15x + 9. A table of values is drawn up as shown below.

x

−2

−1

0

1

2

3

4x3 −8x2 −15x +9

−32 −32 30 9

−4 −8 15 9

0 0 0 9

4 −8 −15 9

32 −32 −30 9

108 −72 −45 9

y

−25

12

9

−10

−21

0

A graph of y = 4x3 − 8x2 − 15x + 9 is shown in Fig. 13.15. The graph crosses the x-axis (where y = 0) at x = −1 12 , x = 12 and x = 3 and these are the solutions to the cubic equation 4x3 − 8x2 − 15x + 9 = 0. The turning points occur at (−0.6, 14.2), which is a maximum, and (2, −21), which is a minimum.

Problem 9. Plot the graph of y = 2x3 − 7x2 + 4x + 4 for values of x between x = −1 and x = 3. Hence determine the roots of the equation 2x3 − 7x2 + 4x + 4 = 0

Graphical solution of equations

A graph of y = 2x3 − 7x2 + 4x + 4 is shown in Fig. 13.16. The graph crosses the x-axis at x = −0.5 and touches the x-axis at x = 2. Hence the solutions of the equation 2x3 − 7x2 + 4x + 4 = 0 are x = −0.5 and x = 2

A table of values is drawn up as shown below. x

−1

−0

1

2

3

2x3 −7x2 +4x +4

−2 −7 −4 4

0 0 0 4

2 −7 4 4

16 −28 8 4

54 −63 12 4

y

−9

4

3

0

7

Now try the following exercise Exercise 52

y

2. By plotting a graph of y = x3 − 2x2 − 5x + 6 between x = −3 and x = 4 solve the equation x3 − 2x2 − 5x + 6 = 0. Determine also the co-ordinates of the turning points and distinguish between them.

14.2 12 8

⫺2

Further problems on solving cubic equations (Answers on page 276)

1. Plot the graph y = 4x3 + 4x2 − 11x − 6 between x = −3 and x = 2 and use the graph to solve the cubic equation 4x3 + 4x2 − 11x − 6 = 0

16

4

y ⫽ 4x 3 ⫺8x 2 ⫺ 15x ⫹ 9

⫺1⫺0.6 0 ⫺4

1

2

3

x

In Problems 3 to 6, solve graphically the cubic equations given, each correct to 2 significant figures. 3. x3 − 1 = 0

⫺8

4. x3 − x2 − 5x + 2 = 0

⫺12

5. x3 − 2x2 = 2x − 2

⫺16

6. 2x3 − x2 − 9.08x + 8.28 = 0

⫺20 ⫺21 ⫺24

7. Show that the cubic equation 8x3 + 36x2 + 54x + 27 = 0 has only one real root and determine its value.

Fig. 13.15 y 8 6

y ⫽ 2x3 ⫺ 7x 2 ⫹ 4x ⫹ 4

4 2 ⫺1

0 ⫺2 ⫺4 ⫺6 ⫺8

Fig. 13.16

101

1

2

3

x

102

Basic Engineering Mathematics

4. The velocity v of a body over varying time intervals t was measured as follows:

Assignment 6 This assignment covers the material contained in Chapters 12 and 13. The marks for each question are shown in brackets at the end of each question. 1. Determine the gradient and intercept on the y-axis for the following equations: a) y = −5x + 2

(b) 3x + 2y + 1 = 0

(5)

2. The equation of a line is 2y = 4x + 7. A table of corresponding values is produced and is as shown below. Complete the table and plot a graph of y against x. Determine the gradient of the graph. (6) x y

−3 −2.5

−2

−1

0

1

2 7.5

3

y 3. Plot the graphs y = 3x + 2 and + x = 6 on the same 2 axes and determine the co-ordinates of their point of intersection. (7)

t seconds v m/s

2 5 7 10 14 17 15.5 17.3 18.5 20.3 22.7 24.5

Plot a graph with velocity vertical and time horizontal. Determine from the graph (a) the gradient, (b) the vertical axis intercept, (c) the equation of the graph, (d) the velocity after 12.5 s, and (e) the time when the velocity is 18 m/s. (9) 5. Solve, correct to 1 decimal place, the quadratic equation 2x2 − 6x − 9 = 0 by plotting values of x from x = −2 to x=5 (7) 6. Plot the graph of y = x3 + 4x2 + x − 6 for values of x between x = −4 and x = 2. Hence determine the roots of the equation x3 + 4x2 + x − 6 = 0 (7) 7. Plot a graph of y = 2x2 from x = −3 to x = +3 and hence solve the equations: (a) 2x2 − 8 = 0

(b) 2x2 − 4x − 6 = 0

(9)

14 Logarithms

14.1 Introduction to logarithms With the use of calculators firmly established, logarithmic tables are now rarely used for calculation. However, the theory of logarithms is important, for there are several scientific and engineering laws that involve the rules of logarithms. If a number y can be written in the form ax , then the index x is called the ‘logarithm of y to the base of a’, i.e.

if y = a

x

then x = loga y

(i) To multiply two numbers: log(A × B) = log A + log B The following may be checked by using a calculator: lg 10 = 1, also

lg 5 + lg 2 = 0.69897 . . . + 0.301029 . . . = 1

Hence

lg (5 × 2) = lg 10 = lg 5 + lg 2

(ii) To divide two numbers: Thus, since 1000 = 103 , then 3 = log10 1000 Check this using the ‘log’ button on your calculator. (a) Logarithms having a base of 10 are called common logarithms and log10 is usually abbreviated to lg. The following values may be checked by using a calculator: lg 17.9 = 1.2528 . . . , lg 462.7 = 2.6652 . . . and lg 0.0173 = −1.7619 … (b) Logarithms having a base of e (where ‘e’ is a mathematical constant approximately equal to 2.7183) are called hyperbolic, Napierian or natural logarithms, and loge is usually abbreviated to ln. The following values may be checked by using a calculator:

log

The following may be checked using a calculator: 5 ln = ln 2.5 = 0.91629 . . . 2 Also ln 5 − ln 2 = 1.60943 . . . − 0.69314 . . . = 0.91629 . . . 5 = ln 5 − ln 2 Hence ln 2 (iii) To raise a number to a power: log An = n log A

ln 3.15 = 1.1474 . . . , ln 362.7 = 5.8935 . . . and ln 0.156 = −1.8578 …

The following may be checked using a calculator: lg 52 = lg 25 = 1.39794 . . .

For more on Napierian logarithms see Chapter 15.

14.2 Laws of logarithms There are three laws of logarithms, which apply to any base:

A = log A − log B B

Also

2 lg 5 = 2 × 0.69897 . . . = 1.39794 . . .

Hence

lg 52 = 2 lg 5

Problem 1. Evaluate (a) log3 9

(b) log10 10

(c) log16 8

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Basic Engineering Mathematics

(a) Let x = log3 9 then 3x = 9 from the definition of a logarithm, i.e. 3x = 32 , from which x = 2. Hence log3 9 = 2 (b) Let x = log10 10 then 10x = 10 from the definition of a logarithm, i.e. 10x = 101 , from which x = 1.

Problem 5. Write log and log 5 to any base.

Hence log10 10 = 1 (which may be checked by a calculator).

log

(c) Let x = log16 8 then 16x = 8, from the definition of a logarithm, i.e. (24 )x = 23 , i.e. 24x = 23 from the laws of indices, from which, 4x = 3 and x = 34

√ √ 8× 4 5 = log 8 + log 4 5 − log 81, by the first 81 and second laws of logarithms = log 23 + log 5(1/4) − log 34

Hence log16 8 = 34 Problem 2.

Evaluate (a) lg 0.001

(a) Let x = lg 0.001 = log10 0.001 10x = 10−3 , from which x = −3

(b) ln e

then

(c) log3

i.e

1 81

10x = 0.001,

i.e.

Hence lg 0.001 = −3 (which may be checked by a calculator). (b) Let x = ln e = loge e then ex = e, i.e. ex = e1 from which x = 1

√ 8× 4 5 in terms of log 2, log 3 81

log

by the laws of indices

√ 4 1 8× 5 = 3 log 2 + log 5 − 4 log 3 81 4 by the third law of logarithms

Problem 6.

Simplify log 64 − log 128 + log 32

64 = 26 , 128 = 27 and 32 = 25 Hence log 64 − log 128 + log 32 = log 26 − log 27 + log 25

Hence ln e = 1 (which may be checked by a calculator). 1 1 1 then 3x = = (c) Let x = log3 = 3−4 , from which 81 81 34 x = −4 1 Hence log3 = −4 81

= 6 log 2 − 7 log 2 + 5 log 2 by the third law of logarithms = 4 log 2

Problem 7.

Evaluate

log 25 − log 125 + 12 log 625 3 log 5

Problem 3. Solve the following equations: (a) lg x = 3 (b) log2 x = 3 (c) log5 x = −2 (a) If lg x = 3 then log10 x = 3 and x = 103 , i.e. x = 1000

log 25 − log 125 + 3 log 5

(c) If log5 x = −2 then x = 5

log 52 − log 53 + 3 log 5

1 2

log 54 4 2

log 5

(b) log 450 in terms of log 2, Problem 8.

(a)

= =

1 1 = 2= 5 25

Problem 4. Write (a) log 30 log 3 and log 5 to any base.

log 625

2 log 5 − 3 log 5 + 3 log 5 1 log 5 1 = = 3 log 5 3

(b) If log2 x = 3 then x = 23 = 8 −2

1 2

log 30 = log(2 × 15) = log(2 × 3 × 5)

Solve the equation:

log(x − 1) + log(x + 1) = 2 log(x + 2)

= log 2 + log 3 + log 5 by the first law of logarithms (b)

log 450 = log(2 × 225) = log(2 × 3 × 75)

log(x − 1) + log(x + 1) = log(x − 1)(x + 1) from the first law of logarithms

= log(2 × 3 × 3 × 25)

= log(x2 − 1)

= log(2 × 32 × 52 )

2 log(x + 2) = log(x + 2)2 = log(x2 + 4x + 4)

= log 2 + log 32 + log 52 by the first law of logarithms i.e

log 450 = log 2 + 2 log 3 + 2 log 5 by the third law of logarithms

Hence if then i.e.

log(x2 − 1) = log(x2 + 4x + 4) (x2 − 1) = x2 + 4x + 4 −1 = 4x + 4

Logarithms

−5 = 4x

i.e.

solve, say, 3x = 27, logarithms to a base of 10 are taken of both sides,

1 5 x = − or −1 4 4

i.e.

i.e.

log10 3x = log10 27

and

x log10 3 = log10 27 by the third law of logarithms.

Now try the following exercise Rearranging gives x = Exercise 53 Further problems on the laws of logarithms (Answers on page 276) In Problems 1 to 11, evaluate the given expression: 1. log10 10 000 1 4. log2 8 7. lg 100 10. log27 3

2. log2 16

3. log5 125

5. log8 2

6. log7 343

8. lg 0.01

9. log4 8

be readily checked.

Problem 9. Solve the equation 2x = 3, correct to 4 significant figures. Taking logarithms to base 10 of both sides of 2x = 3 gives: log10 2x = log10 3

In Problems 12 to 18 solve the equations: 12. log10 x = 4

13. log x = 5

14. log3 x = 2

15. log4 x = −2

16. lg x = −2

17. log8 x = −

i.e.

x log10 2 = log10 3

Rearranging gives: 1 2

x=

4 3

18. ln x = 3 In Problems 19 to 22 write the given expressions in terms of log 2, log 3 and log 5 to any base: 20. log 300 √ 125 × 4 16 22. log √ 4 813

log10 3 0.47712125 . . . = = 1.585 log10 2 0.30102999 . . . correct to 4 significant figures.

Problem 10. Solve the equation 2x + 1 = 32x − 5 correct to 2 decimal places. Taking logarithms to base 10 of both sides gives: log10 2x + 1 = log10 32x − 5 (x + 1) log10 2 = (2x − 5) log10 3

i.e.

x log10 2 + log10 2 = 2x log10 3 − 5 log10 3

Simplify the expressions given in Problems 23 to 25:

x(0.3010) + (0.3010) = 2x(0.4771) − 5(0.4771)

23. log 27 − log 9 + log 81 24. log 64 + log 32 − log 128

i.e.

25. log 8 − log 4 + log 32

Hence

0.3010x + 0.3010 = 0.9542x − 2.3855 2.3855 + 0.3010 = 0.9542x − 0.3010x 2.6865 = 0.6532x

Evaluate the expressions given in Problems 26 and 27: 26. 27.

1 2

log10 27 1.43136 . . . = = 3 which may log10 3 0.4771 . . .

(Note, (log 8/ log 2) is not equal to lg(8/2))

11. ln e2

19. log 60 √ 16 × 4 5 21. log 27

105

log 16 − 13 log 8 log 4

log 9 − log 3 + 12 log 81 2 log 3

Solve the equations given in Problems 28 to 30: 28. log x4 − log x3 = log 5x − log 2x 29. log 2t 3 − log t = log 16 + log t

from which x =

Problem 11. Solve the equation x3.2 = 41.15, correct to 4 significant figures. Taking logarithms to base 10 of both sides gives: log10 x3.2 = log10 41.15

30. 2 log b2 − 3 log b = log 8b − log 4b

14.3 Indicial equations The laws of logarithms may be used to solve certain equations involving powers – called indicial equations. For example, to

2.6865 = 4.11 0.6532 correct to 2 decimal places.

3.2 log10 x = log10 41.15 log10 41.15 = 0.50449 3.2 0.50449 Thus x = antilog 0.50449 = 10 = 3.195 correct to 4 significant figures. Hence

log10 x =

106

Basic Engineering Mathematics

y 2

Now try the following exercise Exercise 54 Indicial equations (Answers on page 276) Solve the following indicial equations for x, each correct to 4 significant figures: 1. 3x = 6.4

2. 2x = 9

3. 2x − 1 = 32x − 1

4. x1.5 = 14.91

5. 25.28 = 4.2x

6. 42x − 1 = 5x + 2

7. x−0.25 = 0.792

8. 0.027x = 3.26

1

0

A graph of y = log10 x is shown in Fig. 14.1 and a graph of y = loge x is shown in Fig. 14.2. Both are seen to be of similar shape; in fact, the same general shape occurs for a logarithm to any base. In general, with a logarithm to any base a, it is noted that: (i) loga 1 = 0 Let loga = x, then ax = 1 from the definition of the logarithm.

2

3

4

5

6

x

⫺1 ⫺2

14.4 Graphs of logarithmic functions

1

6 5 4 3 2 1 0.5 0.2 0.1 x y ⫽ logex 1.79 1.61 1.39 1.10 0.69 0 ⫺0.69 ⫺1.61 ⫺2.30

Fig. 14.2 (ii) loga a = 1 Let loga a = x then ax = a from the definition of a logarithm. If ax = a then x = 1 Hence loga a = 1. (Check with a calculator that log10 10 = 1 and loge e = 1) (iii) loga 0 → −∞

If ax = 1 then x = 0 from the laws of logarithms.

Let loga 0 = x then ax = 0 from the definition of a logarithm.

Hence loga 1 = 0. In the above graphs it is seen that log10 1 = 0 and loge 1 = 0

If ax = 0, and a is a positive real number, then x must approach minus infinity. (For example, check with a calculator, 2−2 = 0.25, 2−20 = 9.54 × 10−7 , 2−200 = 6.22 × 10−61 , and so on.)

y

Hence loga 0 → −∞

0.5 0 1 ⫺0.5

⫺1.0

Fig. 14.1

2

3

x

x 3 2 1 0.5 0.2 0.1 y ⫽ log10x 0.48 0.30 0 ⫺0.30 ⫺0.70 ⫺1.0

15 Exponential functions

15.1 The exponential function An exponential function is one which contains ex , e being a constant called the exponent and having an approximate value of 2.7183. The exponent arises from the natural laws of growth and decay and is used as a base for natural or Napierian logarithms.

e9.32 = 11 159, correct to 5 significant figures e−2.785 = 0.0617291, correct to 7 decimal places Problem 1. Using a calculator, evaluate, correct to 5 significant figures: 5 (a) e2.731 (b) e−3.162 (c) e5.253 3

15.2 Evaluating exponential functions The value of ex may be determined by using: (a) a calculator, or (b) the power series for ex (see Section 15.3), or (c) tables of exponential functions. The most common method of evaluating an exponential function is by using a scientific notation calculator, this now having replaced the use of tables. Most scientific notation calculators contain an ex function which enables all practical values of ex and e−x to be determined, correct to 8 or 9 significant figures. For example: e1 = 2.7182818 e2.4 = 11.023176 e−1.618 = 0.19829489 correct to 8 significant figures. In practical situations the degree of accuracy given by a calculator is often far greater than is appropriate. The accepted convention is that the final result is stated to one significant figure greater than the least significant measured value. Use your calculator to check the following values: e

0.12

= 1.1275, correct to 5 significant figures

e−1.47 = 0.22993, correct to 5 decimal places e−0.431 = 0.6499, correct to 4 decimal places

(a) e2.731 = 15.348227 . . . = 15.348, correct to 5 significant figures. (b) e−3.162 = 0.04234097 . . . = 0.042341, correct to 5 significant figures. 5 5 (c) e5.253 = (191.138825 . . .) = 318.56, correct to 5 signifi3 3 cant figures. Problem 2. Use a calculator to determine the following, each correct to 4 significant figures: 5 7 (a) 3.72e0.18 (b) 53.2e−1.4 (c) e 122 (a) 3.72e0.18 = (3.72)(1.197217 . . .) = 4.454, correct to 4 significant figures. (b) 53.2e−1.4 = (53.2)(0.246596 . . .) = 13.12, correct to 4 significant figures. 5 5 7 e = (1096.6331 . . .) = 44.94, correct to 4 signifi(c) 122 122 cant figures. Problem 3. Evaluate the following correct to 4 decimal places, using a calculator: 0.25 e − e−0.25 (a) 0.0256(e5.21 − e2.49 ) (b) 5 0.25 e + e−0.25

108

Basic Engineering Mathematics

(a) 0.0256(e5.21 − e2.49 )

to 4 significant figures, when l0 = 2.587, θ = 321.7 and α = 1.771 × 10−4

= 0.0256(183.094058 . . . −12.0612761 . . .) = 4.3784, correct to 4 decimal places (b) 5

e0.25 − e−0.25 e0.25 + e−0.25 1.28402541 . . . − 0.77880078 . . . =5 1.28402541 . . . + 0.77880078 . . . 0.5052246 . . . =5 2.0628261 . . . = 1.2246, correct to 4 decimal places

Problem 4. The instantaneous voltage v in a capacitive circuit is related to time t by the equation v = V e−t/CR where V , C and R are constants. Determine v, correct to 4 significant figures, when t = 30 × 10−3 seconds, C = 10 × 10−6 farads, R = 47 × 103 ohms and V = 200 volts. v = V e−t/CR = 200e(−30×10

−3 )/(10×10−6 ×47×103 )

Using a calculator, v = 200e−0.0638297 ... = 200(0.9381646 …)

8. When a chain of length 2L is suspended from two points, 2D metres apart, on the same horizontal level:

√ L + L2 + k 2 D = k ln k Evaluate D when k = 75 m and L = 180 m.

15.3 The power series for ex The value of ex can be calculated to any required degree of accuracy since it is defined in terms of the following power series: ex = 1 + x +

(1)2 (1)3 (1)4 (1)5 (1)6 + + + + 2! 3! 4! 5! 6! (1)7 (1)8 + + ··· + 7! 8! = 1 + 1 + 0.5 + 0.16667 + 0.04167 + 0.00833

e1 = 1 + 1 +

Now try the following exercise Exercise 55 Further problems on evaluating exponential functions (Answers on page 276) In Problems 1 and 2 use a calculator to evaluate the given functions correct to 4 significant figures: (b) e−0.25

(c) e0.92

2. (a) e−1.8

(b) e−0.78

(c) e10

(b) e−2.7483

In Problems 5 and 6, evaluate correct to 5 decimal places. 1 3.4629 e 7

(b) 8.52e−1.2651

6. (a)

5.6823 e−2.1347

(b)

e2.1127 − e−2.1127 2

= 2.71828 i.e.

e = 2.7183 correct to 4 decimal places

(0.05)3 (0.05)2 + 2! 3! (0.05)4 (0.05)5 + + + ··· 4! 5! = 1 + 0.05 + 0.00125 + 0.000020833

e0.05 = 1 + 0.05 +

+ 0.000000260 + 0.000000003

(c) 0.62e4.178

5. (a)

+0.00139 + 0.00020 + 0.00002 + · · ·

The value of e0.05 , correct to say 8 significant figures, is found by substituting x = 0.05 in the power series for ex . Thus

3. Evaluate, correct to 5 significant figures: 6 (a) 3.5e2.8 (b) − e−1.5 (c) 2.16e5.7 5 4. Use a calculator to evaluate the following, correct to 5 significant figures: (a) e1.629

(1)

(where 3! = 3 × 2 × 1 and is called ‘factorial 3’) The series is valid for all values of x. The series is said to converge, i.e. if all the terms are added, an actual value for ex (where x is a real number) is obtained. The more terms that are taken, the closer will be the value of ex to its actual value. The value of the exponent e, correct to say 4 decimal places, may be determined by substituting x = 1 in the power series of equation (1). Thus

= 187.6 volts

1. (a) e4.4

x3 x4 x2 + + + ··· 2! 3! 4!

2.6921

(c)

5e 3e1.1171

(c)

4(e−1.7295 − 1) e3.6817

7. The length of a bar, l, at a temperature θ is given by l = l0 eαθ , where l0 and α are constants. Evaluate l, correct

and by adding, e0.05 = 1.0512711, correct to 8 significant figures In this example, successive terms in the series grow smaller very rapidly and it is relatively easy to determine the value of e0.05 to a high degree of accuracy. However, when x is nearer to unity or larger than unity, a very large number of terms are required for an accurate result.

Exponential functions

= 1 − 1 + 0.5 − 0.166667 + 0.041667

If in the series of equation (1), x is replaced by −x, then −x

e

e−x

−0.008333 + 0.001389 − 0.000198 + · · ·

(−x)3 (−x)2 + + ··· = 1 + (−x) + 2! 3! 2 3 x x − + ··· =1−x+ 2! 3!

= 0.367858 correct to 6 decimal places Hence 3e−1 = (3)(0.367858) = 1.1036 correct to 4 decimal places.

In a similar manner the power series for ex may be used to evaluate any exponential function of the form aekx , where a and k are constants. In the series of equation (1), let x be replaced by kx. Then (kx)2 (kx)3 + + ··· aekx = a 1 + (kx) + 2! 3! 2 (2x)3 (2x) + + ··· Thus 5e2x = 5 1 + (2x) + 2! 3! 2 3 8x 4x + + ··· = 5 1 + 2x + 2 6 4 i.e. 5e2x = 5 1 + 2x + 2x2 + x3 + · · · 3

Problem 5. Determine the value of 5e0.5 , correct to 5 significant figures by using the power series for ex .

x2 x3 x4 + + + ··· 2! 3! 4! (0.5)3 (0.5)2 + e0.5 = 1 + 0.5 + (2)(1) (3)(2)(1) (0.5)5 (0.5)4 + + (4)(3)(2)(1) (5)(4)(3)(2)(1) ex = 1 + x +

Hence

+

e

(0.5)6 (6)(5)(4)(3)(2)(1)

Hence 5e0.5 = 5(1.64872) = 8.2436, correct to 5 significant figures. Problem 6. Determine the value of 3e−1 , correct to 4 decimal places, using the power series for ex .

gives

x3 x4 x5 x2 + + + + ··· 2! 3! 4! 5!

Hence x3 x4 x5 x2 e (x − 1) = 1 + x + + + + + · · · (x2 − 1) 2! 3! 4! 5! x5 x4 + + ··· = x2 + x3 + 2! 3! x2 x3 x4 x5 − 1+x+ + + + + ··· 2! 3! 4! 5!

x

2

when expanded as far as the term in x5

Now try the following exercise Exercise 56

Further problems on the power series for ex (Answers on page 276)

2. Use the power series for ex to determine, correct to 4 significant figures, (a) e2 (b) e−0.3 and check your result by using a calculator.

x3 x4 x2 + + + ··· 2! 3! 4!

e−1 = 1 + (−1) +

ex = 1 + x +

1. Evaluate 5.6e−1 , correct to 4 decimal places, using the power series for ex .

Substituting x = −1 in the power series

2

The power series for ex is

5 11 19 5 1 x = −1 − x + x 2 + x 3 + x 4 + 2 6 24 120

= 1.64872 correct to 6 significant figures

ex = 1 + x +

Expand ex (x2 − 1) as far as the term in x5

x2 x3 ex (x2 − 1) = −1 − x + x2 − + x3 − 2! 3! 4 5 x x4 x5 x − − + + + ··· 2! 4! 3! 5!

+ 0.0026042 + 0.0002604 + 0.0000217 i.e.

Problem 7.

Grouping like terms gives:

= 1 + 0.5 + 0.125 + 0.020833 0.5

109

3

3. Expand (1 − 2x)e2x as far as the term in x4 . 4

(−1) (−1) (−1) + + + ··· 2! 3! 4!

2

4. Expand (2ex )(x1/2 ) to six terms.

110

Basic Engineering Mathematics

15.4 Graphs of exponential functions

y

Values of ex and e−x obtained from a calculator, correct to 2 decimal places, over a range x = −3 to x = 3, are shown in the following table.

y ⫽ 2e0.3x

5

3.87

x −3.0 −2.5 −2.0 −1.5 −1.0 −0.5 0 0.05 0.08 0.14 0.22 0.37 0.61 1.00 ex e−x 20.09 12.18 7.9 4.48 2.72 1.65 1.00

4

3 2

0.5 1.65 0.61

x ex e−x

1.0 2.72 0.37

1.5 4.48 0.22

2.0 7.39 0.14

2.5 12.18 0.08

1.6

3.0 20.09 0.05

1

⫺3

Figure 15.1 shows graphs of y = ex and y = e−x

⫺2

⫺1

0

1

⫺0.74

y

2

x

3

2.2

Fig. 15.2

20 y ⫽ e⫺x

Problem 9. Plot a graph of y = 13 e−2x over the range x = −1.5 to x = 1.5. Determine from the graph the value of y when x = −1.2 and the value of x when y = 1.4

y ⫽ ex 16

A table of values is drawn up as shown below.

12

x −2x e−2x 1 −2x e 3

8

4

⫺3

⫺2

⫺1

0

−1.5 −1.0 −0.5 3 2 1 20.086 7.389 2.718 6.70 2.46 0.91

0 0.5 1.0 1.5 0 −1 −2 −3 1.00 0.368 0.135 0.050 0.33 0.12 0.05 0.02

A graph of 13 e−2x is shown in Fig. 15.3. From the graph, when x = −1.2, y = 3.67 and when y = 1.4, x = −0.72 1

2

3

x y

Fig. 15.1 7 1 y ⫽ e⫺2x 3

Problem 8. Plot a graph of y = 2e0.3x over a range of x = −2 to x = 3. Hence determine the value of y when x = 2.2 and the value of x when y = 1.6

6 5 4 3.67

A table of values is drawn up as shown below. 3

x 0.3x e0.3x 2e0.3x

−3 −2 −1 −0.9 −0.6 −0.3 0.407 0.549 0.741 0.81 1.10 1.48

0 0 1.000 2.00

1 0.3 1.350 2.70

2 0.6 1.822 3.64

3 0.9 2.460 4.92

A graph of y = 2e0.3x is shown plotted in Fig. 15.2. From the graph, when x = 2.2, y = 3.87 and when y = 1.6, x = −0.74

2 1 ⫺0.5 ⫺1.5 ⫺1.0 ⫺1.2 ⫺0.72

Fig. 15.3

1.4

0.5

1.0

1.5

x

Exponential functions

Problem 10. The decay of voltage, v volts, across a capacitor at time t seconds is given by v = 250e−t/3 . Draw a graph showing the natural decay curve over the first 6 seconds. From the graph, find (a) the voltage after 3.4 s, and (b) the time when the voltage is 150 V. A table of values is drawn up as shown below. 0 1.00 250.0

t e−t/3 v = 250e−t/3 t e−t/3 v = 250e−t/3

1 0.7165 179.1

4 0.2636 65.90

2 0.5134 128.4

5 0.1889 47.22

3 0.3679 91.97

3. In a chemical reaction the amount of starting material C cm3 left after t minutes is given by C = 40e−0.006t . Plot a graph of C against t and determine (a) the concentration C after 1 hour, and (b) the time taken for the concentration to decrease by half. 4. The rate at which a body cools is given by θ = 250e−0.05t where the excess of temperature of a body above its surroundings at time t minutes is θ ◦ C. Plot a graph showing the natural decay curve for the first hour of cooling. Hence determine (a) the temperature after 25 minutes, and (b) the time when the temperature is 195◦ C.

15.5 Napierian logarithms

6 0.1353 33.83

The natural decay curve of v = 250e−t/3 is shown in Fig. 15.4.

Logarithms having a base of e are called hyperbolic, Napierian or natural logarithms and the Napierian logarithm of x is written as loge x, or more commonly, ln x.

15.6 Evaluating Napierian logarithms

250 v ⫽ 250e⫺t/3

The value of a Napierian logarithm may be determined by using:

200 Voltage v (volts)

111

(a) a calculator, or (b) a relationship between common and Napierian logarithms, or

150

(c) Napierian logarithm tables. The most common method of evaluating a Napierian logarithm is by a scientific notation calculator, this now having replaced the use of four-figure tables, and also the relationship between common and Napierian logarithms,

100 80 50

loge y = 2.3026 log10 y 0

1 1.5 2 3 3.4 4 Time t (seconds)

5

6

Fig. 15.4

Most scientific notation calculators contain a ‘ln x’ function which displays the value of the Napierian logarithm of a number when the appropriate key is pressed. Using a calculator,

From the graph: (a) when time t = 3.4 s, voltage υ = 80 volts and (b) when voltage υ = 150 volts, time t = 1.5 seconds. Now try the following exercise

and

ln 4.692 = 1.5458589 … = 1.5459, correct to 4 decimal places ln 35.78 = 3.57738907 … = 3.5774, correct to 4 decimal places

Use your calculator to check the following values:

Exercise 57 Further problems on exponential graphs (Answers on page 276)

ln 1.732 = 0.54928, correct to 5 significant figures

1. Plot a graph of y = 3e0.2x over the range x = −3 to x = 3. Hence determine the value of y when x = 1.4 and the value of x when y = 4.5

ln 0.52 = −0.6539, correct to 4 decimal places

2. Plot a graph of y = 12 e−1.5x over a range x = −1.5 to x = 1.5 and hence determine the value of y when x = −0.8 and the value of x when y = 3.5

ln 1 = 0 ln 593 = 6.3852, correct to 5 significant figures ln 1750 = 7.4674, correct to 4 decimal places ln 0.17 = −1.772, correct to 4 significant figures ln 0.00032 = −8.04719, correct to 6 significant figures

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Basic Engineering Mathematics

ln e3 = 3 ln e1 = 1 From the last two examples we can conclude that

(a)

ln e2.5 2.5 =5 = lg 100.5 0.5

(b)

4e2.23 lg 2.23 4(9.29986607 . . .)(0.34830486 . . .) = ln 2.23 0.80200158 . . . = 16.156,

loge ex = x This is useful when solving equations involving exponential functions. For example, to solve e3x = 8, take Napierian logarithms of both sides, which gives ln e3x = ln 8 x=

from which

1 3

ln 8 = 0.6931,

correct to 4 decimal places

Problem 11. Using a calculator evaluate correct to 5 significant figures: (a) ln 47.291

(b) ln 0.06213

(c) 3.2 ln 762.923

(a) ln 47.291 = 3.8563200 . . . = 3.8563, correct to 5 significant figures. (b) ln 0.06213 = −2.7785263 . . . = −2.7785, correct to 5 significant figures. (c) 3.2 ln 762.923 = 3.2(6.6371571 . . .) = 21.239, correct to 5 significant figures.

Problem 12. Use a calculator to evaluate the following, each correct to 5 significant figures: (a)

(a)

(b)

(c)

1 ln 4.7291 4

(b)

ln 7.8693 7.8693

(c)

Problem 14. Solve the equation 7 = 4e−3x to find x, correct to 4 significant figures. Rearranging 7 = 4e−3x gives: 7 = e−3x 4 Taking the reciprocal of both sides gives:

3x = ln 8

i.e.

5.29 ln 24.07 e−0.1762

1 1 ln 4.7291 = (1.5537349 . . .) = 0.38843, correct to 5 sig4 4 nificant figures. ln 7.8693 2.06296911 . . . = = 0.26215, correct to 5 signifi7.8693 7.8693 cant figures. 5.29 ln 24.07 5.29(3.18096625 . . .) = 20.070, correct to = e−0.1762 0.83845027 . . . 5 significant figures.

1 4 = −3x = e3x 7 e Taking Napierian logarithms of both sides gives: 4 ln = ln (e3x ) 7 4 α Since loge e = α, then ln = 3x 7 4 1 1 = (−0.55962) = −0.1865, correct to 4 Hence x = ln 3 7 3 significant figures. Problem 15. Given 20 = 60(1 − e−t/2 ) determine the value of t, correct to 3 significant figures. Rearranging 20 = 60(1 − e−t/2 ) gives: 20 = 1 − e−1/2 60 and 2 20 = 60 3 Taking the reciprocal of both sides gives: e−t/2 = 1 −

3 2 Taking Napierian logarithms of both sides gives: et/2 =

3 2 t 3 = ln 2 2

ln et/2 = ln i.e.

from which, t = 2 ln Problem 13.

Evaluate the following:

ln e2.5 (a) lg 100.5

4e2.23 lg 2.23 (b) ln 2.23

correct to 3 decimal places

(correct to 3 decimal places)

3 = 0.881, correct to 3 significant figures. 2

Problem 16. find x.

Solve the equation 3.72 = ln

5.14 x

to

Exponential functions

From the definition of a logarithm, since 3.72 = e

3.72

5.14 x

then

5.14 = x 5.14 = 5.14e−3.72 e3.72 x = 0.1246, correct to 4 significant figures x=

Rearranging gives: i.e.

(vi) Discharge of a capacitor

113

q = Qe−t/CR

(vii) Atmospheric pressure

p = p0 e−h/c

(viii) Radioactive decay

N = N0 e−λt

(ix) Decay of current in an inductive circuit

i = I e−Rt/L

(x) Growth of current in a capacitive circuit i = I (1 − e−t/CR ) y A

Now try the following exercise

y ⫽ Ae⫺kx

Exercise 58 Further problems on evaluating Napierian logarithms (Answers on page 277) In Problems 1 to 3 use a calculator to evaluate the given functions, correct to 4 decimal places. 1. (a) ln 1.73

(b) ln 5.413

(c) ln 9.412

2. (a) ln 17.3

(b) ln 541.3

(c) ln 9412

3. (a) ln 0.173 (b) ln 0.005413

x

0 y

(c) ln 0.09412

A

In Problems 4 and 5, evaluate correct to 5 significant figures. 4. (a)

1 ln 5.2932 6

(b)

2.946 ln e lg 101.41

(c)

5.62 ln 321.62 e1.2942

y ⫽ A (1⫺e⫺kx )

−0.1629

1.76

5. (a)

ln 82.473 4.829

(b)

5e 2 ln 0.00165

ln 4.8629 − ln 2.4711 5.173 In Problems 6 to 10 solve the given equations, each correct to 4 significant figures. (c)

6. 1.5 = 4e2t

7. 7.83 = 2.91e−1.7x x

9. 5.17 = ln 4.64

0

x

Fig. 15.5 Problem 17. The resistance R of an electrical conductor at temperature θ ◦ C is given by R = R0 eαθ , where α is a constant and R0 = 5 × 103 ohms. Determine the value of α, correct to 4 significant figures, when R = 6 × 103 ohms and θ = 1500◦ C. Also, find the temperature, correct to the nearest degree, when the resistance R is 5.4 × 103 ohms.

8. 16 = 24(1 − e−t/2 ) 1.59 = 2.43 10. 3.72 ln x

15.7 Laws of growth and decay The laws of exponential growth and decay are of the form y = Ae−kx and y = A(1 − e−kx ), where A and k are constants. When plotted, the form of each of these equations is as shown in Fig. 15.5. The laws occur frequently in engineering and science and examples of quantities related by a natural law include: (i) Linear expansion

l = l0 eαθ

(ii) Change in electrical resistance with temperature

Rθ = R0 eαθ

(iii) Tension in belts

T1 = T0 eµθ

(iv) Newton’s law of cooling (v) Biological growth

θ = θ0 e−kt y = y0 ekt

R = eαθ R0 Taking Napierian logarithms of both sides gives:

Transposing R = R0 eαθ gives

ln

R = ln eαθ = αθ R0

Hence 6 × 103 R 1 1 ln ln = θ R0 1500 5 × 103 1 = (0.1823215 . . . ) = 1.215477 . . . × 10−4 1500

α=

Hence α = 1.215 × 10−4 , correct to 4 significant figures.

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Basic Engineering Mathematics

From above, ln

R R 1 = αθ hence θ = ln R0 α R0

When R = 5.4 × 103 , α = 1.215477 . . . × 10−4 and R0 = 5 × 103 5.4 × 103 1 θ= ln 1.215477 . . . × 10−4 5 × 103 =

104 (7.696104 . . . × 10−2 ) 1.215477 . . .

= 633◦ C correct to the nearest degree.

Problem 18. In an experiment involving Newton’s law of cooling, the temperature θ(◦ C) is given by θ = θ0 e−kt . Find the value of constant k when θ0 = 56.6◦ C, θ = 16.5◦ C and t = 83.0 seconds. Transposing θ = θ0 e−kt gives

Taking the reciprocal of both sides gives: et/CR =

Taking Napierian logarithms of both sides gives: t 8.0 = ln CR 8.0 − i 8.0 Hence t = CR ln 8.0 − i 8.0 = (16 × 10−6 )(25 × 103 ) ln 8.0 − 6.0 when i = 6.0 amperes, 400 8.0 i.e. t = 3 ln = 0.4 ln 4.0 10 2.0 = 0.4(1.3862943 . . .) = 0.5545 s

θ = e−kt from which θ0

1 θ0 = −kt = ekt θ e

= 555 ms, to the nearest millisecond A graph of current against time is shown in Fig. 15.6.

Taking Napierian logarithms of both sides gives:

8 i (A)

θ0 = kt ln θ

6

from which,

Hence

5.71

56.6 1 1 θ0 ln k = ln = t θ 83.0 16.5 1 (1.2326486 . . . ) = 83.0 k = 1.485 × 10−2

−t/CR

2

0

= 8.0[1 − e = 8.0(1 − e

−1.25

0.5 0.555

1.0

1.5

t (s)

Fig. 15.6

Problem 20. The temperature θ2 of a winding which is being heated electrically at time t is given by: θ2 = θ1 (1 − e−t/τ ) where θ1 is the temperature (in degrees Celsius) at time t = 0 and τ is a constant. Calculate. (a) θ1 , correct to the nearest degree, when θ2 is 50◦ C, t is 30 s and τ is 60 s

)

0.5/(16×10−6 )(25×103 )

i ⫽ 8.0 (1⫺e⫺t /CR )

4

Problem 19. The current i amperes flowing in a capacitor at time t seconds is given by i = 8.0(1 − e−t/CR ), where the circuit resistance R is 25 × 103 ohms and capacitance C is 16 × 10−6 farads. Determine (a) the current i after 0.5 seconds and (b) the time, to the nearest millisecond, for the current to reach 6.0 A. Sketch the graph of current against time.

(a) Current i = 8.0(1 − e

8.0 8.0 − i

]

)

= 8.0(1 − 0.2865047 . . .) = 8.0(0.7134952 . . .)

(b) the time t, correct to 1 decimal place, for θ2 to be half the value of θ1 (a) Transposing the formula to make θ1 the subject gives: 50 θ2 = (1 − e−t/τ ) 1 − e−30/60 50 50 = = 1 − e−0.5 0.393469 . . .

= 5.71 amperes i = 1 − e−t/CR 8.0 8.0 − i i = from which, e−t/CR = 1 − 8.0 8.0

θ1 =

(b) Transposing i = 8.0(1 − e−t/CR ) gives

i.e.

θ 1 = 127◦ C, correct to the nearest degree.

Exponential functions

(b) Transposing to make t the subject of the formula gives: θ2 = 1 − e−t/τ θ1 θ2 from which, e−t/τ = 1 − θ1 θ2 t Hence − = ln 1 − θ1 τ θ2 i.e. t = −τ ln 1 − θ1 Since

5. The temperature θ2◦ C of an electrical conductor at time t seconds is given by θ2 = θ1 (1 − e−t/T ), where θ1 is the initial temperature and T seconds is a constant. Determine (a) θ2 when θ1 = 159.9◦ C, t = 30 s and T = 80 s, and (b) the time t for θ2 to fall to half the value of θ1 if T remains at 80 s. 6. A belt is in contact with a pulley for a sector of θ = 1.12 radians and the coefficient of friction between these two surfaces is µ = 0.26. Determine the tension on the taut side of the belt, T newtons, when tension on the slack side is given by T0 = 22.7 newtons, given that these quantities are related by the law T = T0 eµθ

1 θ2 = θ 1 2

1 t = −60 ln 1 − = −60 ln 0.5 = 41.59 s 2

Hence the time for the temperature θ 2 to be one half of the value of θ 1 is 41.6 s, correct to 1 decimal place.

Now try the following exercise Exercise 59 Further problems on the laws of growth and decay (Answers on page 277) 1. The temperature, T ◦ C, of a cooling object varies with time, t minutes, according to the equation: T = 150 e−0.04t Determine the temperature (b) t = 10 minutes.

115

when

(a)

t = 0,

2. The pressure p pascals at height h metres above ground level is given by p = p0 e−h/C , where p0 is the pressure at ground level and C is a constant. Find pressure p when p0 = 1.012 × 105 Pa, height h = 1420 m and C = 71500. 3. The voltage drop, v volts, across an inductor L henrys at time t seconds is given by v = 200e−Rt/L , where R = 150 and L = 12.5 × 10−3 H. Determine (a) the voltage when t = 160 × 10−6 s, and (b) the time for the voltage to reach 85 V. 4. The length l metres of a metal bar at temperature t ◦ C is given by l = l0 eαt , where l0 and α are constants. Determine (a) the value of l when l0 = 1.894, α = 2.038 × 10−4 and t = 250◦ C, and (b) the value of l0 when l = 2.416, t = 310◦ C and α = 1.682 × 10−4

7. The instantaneous current i at time t is given by: i = 10e−t/CR when a capacitor is being charged. The capacitance C is 7 × 10−6 farads and the resistance R is 0.3 × 106 ohms. Determine: (a) the instantaneous current when t is 2.5 seconds, and (b) the time for the instantaneous current to fall to 5 amperes. Sketch a curve of current against time from t = 0 to t = 6 seconds. 8. The amount of product x (in mol/cm3 ) found in a chemical reaction starting with 2.5 mol/cm3 of reactant is given by x = 2.5(1 − e−4t ) where t is the time, in minutes, to form product x. Plot a graph at 30 second intervals up to 2.5 minutes and determine x after 1 minute. 9. The current i flowing in a capacitor at time t is given by: i = 12.5(1 − e−t/CR ) where resistance R is 30 kilohms and the capacitance C is 20 microfarads. Determine (a) the current flowing after 0.5 seconds, and (b) the time for the current to reach 10 amperes. 10. The amount A after n years of a sum invested P is given by the compound interest law: A = Pe−rn/100 when the per unit interest rate r is added continuously. Determine, correct to the nearest pound, the amount after 8 years for a sum of £1500 invested if the interest rate is 6% per annum.

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Basic Engineering Mathematics

5. Use the power series for ex to evaluate e1.5 , correct to 4 significant figures. (4)

Assignment 7 This assignment covers the material contained in Chapters 14 and 15. The marks for each question are shown in brackets at the end of each question. 1. Evaluate log16 8 2. Solve

(3)

(a) log3 x = −2 (b) log 2x2 + log x = log 32 − log x

(6)

3. Solve the following equations, each correct to 3 significant figures: (a) 2x = 5.5 (b) 32t−1 = 7t+2 (c) 3e2x = 4.2

(11)

4. Evaluate the following, each correct to 4 significant figures: (a) e−0.683

(b) 1.34e2.16

(c)

5(e−2.73 − 1) e1.68

(6)

6. Expand xe3x to six terms.

(5)

7. Plot a graph of y = 12 e−1.2x over the range x = −2 to x = +1 and hence determine, correct to 1 decimal place, (a) the value of y when x = −0.75, and (b) the value of x when y = 4.0 (6) 8. Evaluate the following, each correct to 3 decimal places: ln 3.68 − ln 2.91 (a) ln 462.9 (b) ln 0.0753 (c) 4.63 (6) 9. Two quantities x and y are related by the equation y = ae−kx , where a and k are constants. Determine, correct to 1 decimal place, the value of y when a = 2.114, k = −3.20 and x = 1.429 (3)

16 Reduction of non-linear laws to linear form

16.1 Determination of law Frequently, the relationship between two variables, say x and y, is not a linear one, i.e. when x is plotted against y a curve results. In such cases the non-linear equation may be modified to the linear form, y = mx + c, so that the constants, and thus the law relating the variables can be determined. This technique is called ‘determination of law’. Some examples of the reduction of equations to linear form include: (i) y = ax2 + b compares with Y = mX + c, where m = a, c = b and X = x2 . Hence y is plotted vertically against x2 horizontally to produce a straight line graph of gradient ‘a’ and y-axis intercept ‘b’ a (ii) y = + b x 1 y is plotted vertically against horizontally to produce a x straight line graph of gradient ‘a’ and y-axis intercept ‘b’ (iii) y = ax2 + bx y = ax + b. x y Comparing with Y = mX + c shows that is plotted vertix cally against x horizontally to produce a straight line graph y of gradient ‘a’ and axis intercept ‘b’ x

If y is plotted against x a curve results and it is not possible to determine the values of constants a and b from the curve. Comparing y = ax2 + b with Y = mX + c shows that y is to be plotted vertically against x2 horizontally. A table of values is drawn up as shown below. x x2 y

1 1 9.8

2 4 15.2

3 9 24.2

4 16 36.5

5 25 53.0

A graph of y against x2 is shown in Fig. 16.1, with the best straight line drawn through the points. Since a straight line graph results, the law is verified. y 53 50

A

40

30

Dividing both sides by x gives

Problem 1. Experimental values of x and y, shown below, are believed to be related by the law y = ax2 + b. By plotting a suitable graph verify this law and determine approximate values of a and b.

20 17

B

C

10 8

0

5

10

15

20

25

Fig. 16.1 x y

1 9.8

2 15.2

3 24.2

4 36.5

5 53.0

From the graph, gradient a =

AB 53 − 17 36 = = = 1.8 BC 25 − 5 20

x2

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Basic Engineering Mathematics

and the y-axis intercept, b = 8.0 Hence the law of the graph is y = 1.8x 2 + 8.0

35

Problem 2. Values of load L newtons and distance d metres obtained experimentally are shown in the following table.

31 30

A

25

Load, L N distance, d m

32.3 0.75

29.6 0.37

Load, L N distance, d m

18.3 0.12

12.8 0.09

27.0 0.24 10.0 0.08

23.2 0.17

L 20

6.4 0.07

15

Verify that load and distance are related by a law of the a form L = + b and determine approximate values of a and d b. Hence calculate the load when the distance is 0.20 m and the distance when the load is 20 N. 1 a Comparing L = + b i.e. L = a + b with Y = mX + c d d 1 shows that L is to be plotted vertically against horizontally. d Another table of values is drawn up as shown below. L d 1 d

32.3 29.6 27.0 23.2 18.3 12.8 10.0 0.75 0.37 0.24 0.17 0.12 0.09 0.08 1.33

2.70

4.17

5.88

6.4 0.07

8.33 11.11 12.50 14.29

1 is shown in Fig. 16.2. A straight line can d be drawn through the points, which verifies that load and distance a are related by a law of the form L = + b. d AB 31 − 11 = Gradient of straight line, a = BC 2 − 12 20 = −2 = −10 L-axis intercept, b = 35

A graph of L against

2 Hence the law of the graph is L = − + 35 d When the distance d = 0.20 m, load L =

−2 + 35 = 25.0 N 0.20

Rearranging L = −

2 + 35 gives d

2 = 35 − L d

and

d=

2 35 − L

Hence when the load L = 20 N, distance d =

2 2 = = 0.13 m 35 − 20 15

11 10

B

C

5

0

2

4

6

8 1 d

10

12

14

Fig. 16.2

Problem 3. The solubility s of potassium chlorate is shown by the following table: t◦C s

10 20 30 40 50 60 80 100 4.9 7.6 11.1 15.4 20.4 26.4 40.6 58.0

The relationship between s and t is thought to be of the form s = 3 + at + bt 2 . Plot a graph to test the supposition and use the graph to find approximate values of a and b. Hence calculate the solubility of potassium chlorate at 70◦ C.

Rearranging s = 3 + at + bt 2 gives s − 3 = at + bt 2 and s−3 = a + bt t

or

s−3 = bt + a t

s−3 is to be t plotted vertically and t horizontally. Another table of values is drawn up as shown below.

which is of the form Y = mX + c, showing that

t 10 20 30 40 50 60 80 100 s 4.9 7.6 11.1 15.4 20.4 26.4 40.6 58.0 s−3 0.19 0.23 0.27 0.31 0.35 0.39 0.47 0.55 t s−3 against t is shown plotted in Fig. 16.3. t A straight line fits the points which shows that s and t are related by s = 3 + at + bt 2 . Gradient of straight line,

A graph of

b=

0.39 − 0.19 0.20 AB = = = 0.004 BC 60 − 10 50

Reduction of non-linear laws to linear form

and find the approximate values for a and b. Determine the cross-sectional area needed for a resistance reading of 0.50 ohms.

0.6 0.5

s⫺3 t

7. Corresponding experimental values of two quantities x and y are given below.

0.4 0.39

A

x y

0.3 0.2 0.19 0.15 0.1

119

0

20

40

60 80 t °C

100

Fig. 16.3 Vertical axis intercept, a = 0.15 Hence the law of the graph is s = 3 + 0.15t + 0.004t 2 The solubility of potassium chlorate at 70◦ C is given by 2

= 3 + 10.5 + 19.6 = 33.1

0.89 1.75

9.0 169.0

0.76 2.04

2.0 2.8 3.6 4.2 4.8 475 339 264 226 198

9. The following results give corresponding values of two quantities x and y which are believed to be related by a law of the form y = ax2 + bx where a and b are constants. 33.86 3.4

55.54 5.2

72.80 6.5

84.10 7.3

111.4 9.1

168.1 12.4

Hence determine (i) the value of y when x is 8.0 and (ii) the value of x when y is 146.5.

In Problems 1 to 5, x and y are two related variables and all other letters denote constants. For the stated laws to be verified it is necessary to plot graphs of the variables in a modified form. State for each (a) what should be plotted on the vertical axis, (b) what should be plotted on the horizontal axis, (c) the gradient and (d) the vertical axis intercept. √ 2. y − a = b x 1. y = d + cx2 f 4. y − cx = bx2 3. y − e = x a 5. y = + bx x 6. In an experiment the resistance of wire is measured for wires of different diameters with the following results. 1.14 1.42

7.5 119.5

Verify the law and determine approximate values of a and b.

Exercise 60 Further problems on reducing non-linear laws to linear form (Answers on page 277)

1.64 1.10

6.0 79.0

It is believed that the relationship between load and span is L = c/d, where c is a constant. Determine (a) the value of constant c and (b) the safe load for a span of 3.0 m.

y x

Now try the following exercise

R ohms d mm

4.5 47.5

8. Experimental results of the safe load L kN, applied to girders of varying spans, d m, are shown below. Span, d m Load, L kN

s = 3 + 0.15(70) + 0.004(70)

3.0 25.0

By plotting a suitable graph verify that y and x are connected by a law of the form y = kx2 + c, where k and c are constants. Determine the law of the graph and hence find the value of x when y is 60.0

B

C

1.5 11.5

0.63 2.56

It is thought that R is related to d by the law R = (a/d 2 ) + b, where a and b are constants. Verify this

16.2 Determination of law involving logarithms Examples of reduction of equations to linear form involving logarithms include: (i) y = axn Taking logarithms to a base of 10 of both sides gives: lg y = lg (axn ) = lg a + lg xn i.e. lg y = n lg x + lg a by the laws of logarithms which compares with Y = mX + c and shows that lg y is plotted vertically against lg x horizontally to produce a straight line graph of gradient n and lg y-axis intercept lg a

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Basic Engineering Mathematics

(ii) y = abx Taking logarithms to a base of 10 of the both sides gives:

3.0 2.98

lg y = lg(abx ) i.e. lg y = lg a + lg bx

2.78

i.e. lg y = x lg b + lg a or

A D

by the laws of logarithms lg P

lg y = ( lg b)x + lg a

2.5

which compares with Y = mX + c and shows that lg y is plotted vertically against x horizontally to produce a straight line graph of gradient lg b and lg y-axis intercept lg a

2.18

(iii) y = aebx

2.0 0.30 0.40

Taking logarithms to a base of e of both sides gives: ln y = ln(aebx ) i.e. ln y = ln a + bx ln e i.e. ln y = bx + ln a

ln e = 1

n=

and shows that ln y is plotted vertically against x horizontally to produce a straight line graph of gradient b and ln y-axis intercept ln a

Problem 4. The current flowing in, and the power dissipated by, a resistor are measured experimentally for various values and the results are as shown below. 3.6 311

4.1 403

5.6 753

6.8 1110

Show that the law relating current and power is of the form P = RI n , where R and n are constants, and determine the law. Taking logarithms to a base of 10 of both sides of P = RI n gives: lg P = lg(RI n ) = lg R + lg I n = lg R + n lg I by the laws of logarithms, i.e. lg P = n lg I + lg R, which is of the form Y = mX + c, showing that lg P is to be plotted vertically against lg I horizontally. A table of values for lg I and lg P is drawn up as shown below. 2.2 0.342 116 2.064

3.6 0.556 311 2.493

0.70

0.80

0.90

Gradient of straight line,

since

2.2 116

0.60 lg l

which compares with Y = mX + c

Current, I amperes Power, P watts

0.50

Fig. 16.4

i.e. ln y = ln a + ln ebx

I lg I P lg P

B

C

4.1 0.613 403 2.605

5.6 0.748 753 2.877

6.8 0.833 1110 3.045

A graph of lg P against lg I is shown in Fig. 16.4 and since a straight line results the law P = RI n is verified.

2.98 − 2.18 0.80 AB = = =2 BC 0.8 − 0.4 0.4

It is not possible to determine the vertical axis intercept on sight since the horizontal axis scale does not start at zero. Selecting any point from the graph, say point D, where lg I = 0.70 and lg P = 2.78, and substituting values into lg P = n lg I + lg R gives 2.78 = (2)(0.70) + lg R from which lg R = 2.78 − 1.40 = 1.38 Hence

R = antilog 1.38 (= 101.38 ) = 24.0

Hence the law of the graph is P = 24.0 I 2 Problem 5. The periodic time, T , of oscillation of a pendulum is believed to be related to its length, l, by a law of the form T = kl n , where k and n are constants. Values of T were measured for various lengths of the pendulum and the results are as shown below. Periodic time, T s Length, l m

1.0 1.3 1.5 1.8 2.0 2.3 0.25 0.42 0.56 0.81 1.0 1.32

Show that the law is true and determine the approximate values of k and n. Hence find the periodic time when the length of the pendulum is 0.75 m. From para (i), if T = kl n then lg T = n lg l + lg k and comparing with Y = mX + c

Reduction of non-linear laws to linear form

shows that lg T is plotted vertically against lg l horizontally. A table of values for lg T and lg l is drawn up as shown below. T lg T l lg l

1.0 1.3 1.5 1.8 0 0.114 0.176 0.255 0.25 0.42 0.56 0.81 −0.602 −0.377 −0.252 −0.092

2.0 0.301 1.0 0

2.3 0.362 1.32 0.121

A graph of lg T against lg l is shown in Fig. 16.5 and the law T = kl n is true since a straight line results.

lg T

lg y = (lgb)x + lg a and comparing with Y = mX + c shows that lg y is plotted vertically and x horizontally. Another table is drawn up as shown below. 0 5.0 0.70

0.6 9.67 0.99

1.2 18.7 1.27

1.8 36.1 1.56

2.4 69.8 1.84

3.0 135.0 2.13

A graph of lg y against x is shown in Fig. 16.6 and since a straight line results, the law y = abx is verified.

0.30 A

From para (ii), if y = abx then

x y lg y

0.40

121

0.25

2.50 0.20

2.13 2.00

A

C

lg y

0.10 0.05

B

1.50 ⫺0.60 ⫺0.50 ⫺0.40 ⫺0.30 ⫺0.20 ⫺0.10

0

0.10

0.20

lg I

1.17

B

C

Fig. 16.5

1.00

From the graph, gradient of straight line, n=

0.25 − 0.05 0.20 1 AB = = = BC −0.10 − (−0.50) 0.40 2

0.70 0.50 0

1.0

2.0

3.0

x

Vertical axis intercept, lg k = 0.30 Hence k = antilog 0.30 (= 100.30 ) = 2.0

√ Hence the law of the graph is T = 2.0 l 1/2 or T = 2.0 l √ When length l = 0.75 m then T = 2.0 0.75 = 1.73 s Problem 6. Quantities x and y are believed to be related by a law of the form y = abx , where a and b are constants. Values of x and corresponding values of y are: x y

0 5.0

0.6 9.67

1.2 18.7

1.8 36.1

2.4 69.8

3.0 135.0

Verify the law and determine the approximate values of a and b. Hence determine (a) the value of y when x is 2.1 and (b) the value of x when y is 100

Fig. 16.6 Gradient of straight line, lg b =

2.13 − 1.17 0.96 AB = = = 0.48 BC 3.0 − 1.0 2.0

Hence b = antilog 0.48 (= 100.48 ) = 3.0, correct to 2 significant figures. Vertical axis intercept, lg a = 0.70, from which a = antilog 0.70 (= 100.70 ) = 5.0, correct to 2 significant figures Hence the law of the graph is y = 5.0(3.0)x

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Basic Engineering Mathematics

(a) When x = 2.1, y = 5.0(3.0)2.1 = 50.2 (b) When y = 100, 100 = 5.0(3.0)x ,

A

5.0

from which 100/5.0 = (3.0)x , 20 = (3.0)x

i.e.

4.0

Taking logarithms of both sides gives lg 20 = lg(3.0) = x lg 3.0

ln i

Hence x =

1.3010 lg 20 = = 2.73 lg 3.0 0.4771

2.0

Problem 7. The current i mA flowing in a capacitor which is being discharged varies with time t ms as shown below. i mA t ms

203 100

61.14 160

22.49 210

6.13 275

2.49 320

0.615 390

Show that these results are related by a law of the form i = I et/T , where I and T are constants. Determine the approximate values of I and T . Taking Napierian logarithms of both sides of i = I et/T gives ln i = ln (I et/T ) = ln I + ln et/T

160 61.14 4.11

210 22.49 3.11

275 6.13 1.81

320 2.49 0.91

390 0.615 −0.49

A graph of ln i against t is shown in Fig. 16.7 and since a straight line results the law i = I et/T is verified. Gradient of straight line, 1 AB 5.30 − 1.30 4.0 = = = = −0.02 T BC 100 − 300 −200 1 = −50 Hence T = −0.02 Selecting any point on the graph, say point where t = 200 and D, 1 t + ln I gives ln i = 3.31, and substituting into ln i = T 1 (200) + ln I 50 ln I = 3.31 + 4.0 = 7.31

3.31 = − from which

C

B

0

100

200

300

400 t (ms)

⫺1.0

Fig. 16.7

and I = antilog 7.31 (= e7.31 ) = 1495 or 1500 correct to 3 significant figures.

Now try the following exercise

which compares with y = mx + c, showing that ln i is plotted vertically against t horizontally. (For methods of evaluating Napierian logarithms see Chapter 15.) Another table of values is drawn up as shown below. 100 203 5.31

1.30 1.0

Hence the law of the graph is i = 1500e−t/50

t (since ln e = 1) i.e. ln i = ln I + T 1 t + ln I or ln i = T

t i ln i

D (200, 3.31)

3.31 3.0

x

Exercise 61

Further problems on reducing non-linear laws to linear form (Answers on page 277)

In Problems 1 to 3, x and y are two related variables and all other letters denote constants. For the stated laws to be verified it is necessary to plot graphs of the variables in a modified form. State for each (a) what should be plotted on the vertical axis, (b) what should be plotted on the horizontal axis, (c) the gradient and (d) the vertical axis intercept. 1. y = bax

2. y = kxl

3.

y = enx m

4. The luminosity I of a lamp varies with the applied voltage V and the relationship between I and V is thought to be I = kV n . Experimental results obtained are: I candelas V volts

1.92 4.32 9.72 15.87 23.52 30.72 40 60 90 115 140 160

Verify that the law is true and determine the law of the graph. Determine also the luminosity when 75 V is applied across the lamp.

Reduction of non-linear laws to linear form

5. The head of pressure h and the flow velocity v are measured and are believed to be connected by the law v = ahb , where a and b are constants. The results are as shown below. h v

10.6 9.77

13.4 11.0

17.2 12.44

24.6 14.88

29.3 16.24

Verify that the law is true and determine values of a and b 6. Experimental values of x and y are measured as follows.

x y

0.4 8.35

0.9 13.47

1.2 17.94

2.3 51.32

3.8 215.20

The law relating x and y is believed to be of the form y = abx , where a and b are constants. Determine the approximate values of a and b. Hence find the value of y when x is 2.0 and the value of x when y is 100 7. The activity of a mixture of radioactive isotope is believed to vary according to the law R = R0 t −c , where R0 and c are constants.

123

Experimental results are shown below. R t

9.72 2

2.65 5

1.15 9

0.47 17

0.32 22

0.23 28

Verify that the law is true and determine approximate values of R0 and c. 8. Determine the law of the form y = aekx which relates the following values. y x

0.0306 −4.0

0.285 5.3

0.841 5.21 9.8 17.4

173.2 32.0

1181 40.0

9. The tension T in a belt passing round a pulley wheel and in contact with the pulley over an angle of θ radians is given by T = T0 eµθ , where T0 and µ are constants. Experimental results obtained are: T newtons θ radians

47.9 52.8 60.3 70.1 1.12 1.48 1.97 2.53

80.9 3.06

Determine approximate values of T0 and µ. Hence find the tension when θ is 2.25 radians and the value of θ when the tension is 50.0 newtons.

17 Graphs with logarithmic scales

17.1 Logarithmic scales Graph paper is available where the scale markings along the horizontal and vertical axes are proportional to the logarithms of the numbers. Such graph paper is called log–log graph paper. 1

2

3

4

5

6

7 8 9 10

Fig. 17.1 100

A 10

y ⫽ ax b

y

A logarithmic scale is shown in Fig. 17.1 where the distance between, say 1 and 2, is proportional to lg 2 − lg 1, i.e. 0.3010 of the total distance from 1 to 10. Similarly, the distance between 7 and 8 is proportional to lg 8 − lg 7, i.e. 0.05799 of the total distance from 1 to 10. Thus the distance between markings progressively decreases as the numbers increase from 1 to 10. With log–log graph paper the scale markings are from 1 to 9, and this pattern can be repeated several times. The number of times the pattern of markings is repeated on an axis signifies the number of cycles. When the vertical axis has, say, 3 sets of values from 1 to 9, and the horizontal axis has, say, 2 sets of values from 1 to 9, then this log–log graph paper is called ‘log 3 cycle × 2 cycle’ (see Fig. 17.2). Many different arrangements are available ranging from ‘log 1 cycle × 1 cycle’ through to ‘log 5 cycle × 5 cycle’. To depict a set of values, say, from 0.4 to 161, on an axis of log–log graph paper, 4 cycles are required, from 0.1 to 1, 1 to 10, 10 to 100 and 100 to 1000.

17.2 Graphs of the form y = ax n Taking logarithms to a base of 10 of both sides of y = axn gives:

1.0

lg y = lg(axn )

B

C

= lg a + lg xn i.e. which compares with

0.1

Fig. 17.2

1.0 x

10

lg y = n lg x + lg a Y = mX + c

Thus, by plotting lg y vertically against lg x horizontally, a straight line results, i.e. the equation y = axn is reduced to linear form. With log–log graph paper available x and y may be plotted directly, without having first to determine their logarithms, as shown in Chapter 16.

Graphs with logarithmic scales

Problem 1. Experimental values of two related quantities x and y are shown below: x y

0.41 0.45

0.63 1.21

0.92 2.89

1.36 7.10

2.17 20.79

Taking logarithms of both sides gives lg 24.643 = b lg 4, i.e. lg 24.643 lg 4 = 2.3, correct to 2 significant figures.

b=

3.95 82.46

The law relating x and y is believed to be y = axb , where a and b are constants. Verify that this law is true and determine the approximate values of a and b.

If y = ax then lg y = b lg x + lg a, from above, which is of the form Y = mX + c, showing that to produce a straight line graph lg y is plotted vertically against lg x horizontally. x and y may be plotted directly on to log–log graph paper as shown in Fig. 17.2. The values of y range from 0.45 to 82.46 and 3 cycles are needed (i.e. 0.1 to 1, 1 to 10 and 10 to 100). The values of x range from 0.41 to 3.95 and 2 cycles are needed (i.e. 0.1 to 1 and 1 to 10). Hence ‘log 3 cycle × 2 cycle’ is used as shown in Fig. 17.2 where the axes are marked and the points plotted. Since the points lie on a straight line the law y = axb is verified. b

To evaluate constants a and b: Method 1. Any two points on the straight line, say points A and C, are selected, and AB and BC are measured (say in centimetres). Then, gradient, b =

125

AB 11.5 units = = 2.3 BC 5 units

Substituting b = 2.3 in equation (1) gives: 17.25 = a(2)2.3 , i.e. 17.25 17.25 = (2)2.3 4.925 = 3.5, correct to 2 significant figures.

a=

Hence the law of the graph is: y = 3.5x2.3

Problem 2. The power dissipated by a resistor was measured for varying values of current flowing in the resistor and the results are as shown: Current, I amperes Power, P watts

1.4 4.7 6.8 9.1 11.2 13.1 49 552 1156 2070 3136 4290

Prove that the law relating current and power is of the form P = RI n , where R and n are constants, and determine the law. Hence calculate the power when the current is 12 amperes and the current when the power is 1000 watts.

Since lg y = b lg x + lg a, when x = 1, lg x = 0 and lg y = lg a. The straight line crosses the ordinate x = 1.0 at y = 3.5. Hence lg a = lg 3.5, i.e. a = 3.5 Method 2. Any two points on the straight line, say points A and C, are selected. A has coordinates (2, 17.25) and C has coordinates (0.5, 0.7). Since

y = axb then 17.25 = a(2)b 0.7 = a(0.5)b

and

(1) (2)

i.e. two simultaneous equations are produced and may be solved for a and b.

Since P = RI n then lg P = nlg l + lg R, which is of the form Y = mX + c, showing that to produce a straight line graph lg P is plotted vertically against lg I horizontally. Power values range from 49 to 4290, hence 3 cycles of log–log graph paper are needed (10 to 100, 100 to 1000 and 1000 to 10 000). Current values range from 1.4 to 11.2, hence 2 cycles of log–log graph paper are needed (1 to 10 and 10 to 100). Thus ‘log 3 cycles × 2 cycles’ is used as shown in Fig. 17.3 (or, if not available, graph paper having a larger number of cycles per axis can be used). The co-ordinates are plotted and a straight line results which proves that the law relating current and power is of the form P = RI n . Gradient of straight line, n=

Dividing equation (1) by equation (2) to eliminate a gives: 17.25 (2)b = = 0.7 (0.5)b i.e.

24.643 = (4)b

2 0.5

b

AB 14 units = =2 BC 7 units

At point C, I = 2 and P = 100. Substituting these values into P = RI n gives: 100 = R(2)2 . Hence R = 100/(2)2 = 25 which may have been found from the intercept on the I = 1.0 axis in Fig. 17.3.

126

Basic Engineering Mathematics

Since p = cv n , then lg p = n lg v + lg c, which is of the form Y = mX + c, showing that to produce a straight line graph lg p is plotted vertically against lg v horizontally. The co-ordinates are plotted on ‘log 3 cycle × 2 cycle’ graph paper as shown in Fig. 17.4. With the data expressed in standard form, the axes are marked in standard form also. Since a straight line results the law p = cv n is verified.

10000 A

1000 Power, P watts

1 ⫻ 108 P⫽ Rl n

A B

C

10 1.0

1 ⫻ 107

10 Current, l amperes

100

Pressure, p Pascals

100

p ⫽ cv n

1 ⫻ 106

Fig. 17.3 C

B

Hence the law of the graph is P = 25I 2 When current I = 12, power P = 25(12)2 = 3600 watts (which may be read from the graph). When power P = 1000, 1000 = 25I 2 . Hence from which,

1000 = 40, 25 √ I = 40 = 6.32 A

I2 =

p pascals v m3

2.28 × 105 3.2 × 10−2

1 ⫻ 10⫺1

The straight line has a negative gradient and the value of the gradient is given by:

8.04 × 105 1.3 × 10−2

5.05 × 106 3.5 × 10−3

1 ⫻ 10⫺2 Volume, v m3

Fig. 17.4

Problem 3. The pressure p and volume v of a gas are believed to be related by a law of the form p = cv n , where c and n are constants. Experimental values of p and corresponding values of v obtained in a laboratory are: p pascals v m3

1 ⫻ 105 1 ⫻ 10⫺3

2.03 × 106 6.7 × 10−3

1.82 × 107 1.4 × 10−3

Verify that the law is true and determine approximate values of c and n.

14 units AB = = 1.4, BC 10 units hence n = −1.4 Selecting any point on the straight line, say point C, having co-ordinates (2.63 × 10−2 , 3 × 105 ), and substituting these values in p = cv n gives: 3×105 = c(2.63×10−2 )−1.4 Hence

c=

3×105 3×105 = (2.63×10−2 )−1.4 (0.0263)−1.4

3×105 1.63×102 = 1840, correct to 3 significant figures. =

Graphs with logarithmic scales

called log–linear graph paper, and is specified by the number of cycles on the logarithmic scale. For example, graph paper having 3 cycles on the logarithmic scale is called ‘log 3 cycle × linear’ graph paper.

Hence the law of the graph is: p = 1840υ −1.4

pυ 1.4 = 1840

or

Now try the following exercise Exercise 62 Further problems on graphs of the form y = ax n (Answers on page 277) 1. Quantities x and y are believed to be related by a law of the form y = axn , where a and n are constants. Experimental values of x and corresponding values of y are: x y

0.8 8

2.3 54

5.4 250

11.5 974

21.6 3028

42.9 10 410

Show that the law is true and determine the values of a and n. Hence determine the value of y when x is 7.5 and the value of x when y is 5000. 2. Show from the following results of voltage V and admittance Y of an electrical circuit that the law connecting the quantities is of the form V = kY n , and determine the values of k and n. Voltage, V volts Admittance Y siemens

127

2.88

2.05

1.60

1.22

0.96

0.52

0.73

0.94

1.23

1.57

Problem 4. Experimental values of quantities x and y are believed to be related by a law of the form y = abx , where a and b are constants. The values of x and corresponding values of y are: x y

0.7 18.4

1.4 45.1

2.1 111

2.9 308

3.7 858

4.3 1850

Verify the law and determine the approximate values of a and b. Hence evaluate (i) the value of y when x is 2.5, and (ii) the value of x when y is 1200. Since y = abx then lg y = (lg b) x + lg a (from above), which is of the form Y = mX + c, showing that to produce a straight line graph lg y is plotted vertically against x horizontally. Using log– linear graph paper, values of x are marked on the horizontal scale to cover the range 0.7 to 4.3. Values of y range from 18.4 to 1850 and 3 cycles are needed (i.e. 10 to 100, 100 to 1000 and 1000 to 10 000). Thus ‘log 3 cycles × linear’graph paper is used as shown in Fig. 17.5. A straight line is drawn through the co-ordinates, hence the law y = abx is verified. 10000

3. Quantities x and y are believed to be related by a law of the form y = mnx . The values of x and corresponding values of y are: x y

0 1.0

0.5 3.2

1.0 10

1.5 31.6

2.0 100

2.5 316

3.0 1000

A

1000 y ⫽ ab x

Verify the law and find the values of m and n.

17.3

Graphs of the form y = abx

Taking logarithms to a base of 10 of both sides of y = abx gives:

100

B

C

lg y = lg(abx ) = lg a + lg bx = lg a + xlg b i.e. which compares with

lg y = (lg b)x + lg a Y = mX + c

Thus, by plotting lg y vertically against x horizontally a straight line results, i.e. the graph y = abx is reduced to linear form. In this case, graph paper having a linear horizontal scale and a logarithmic vertical scale may be used. This type of graph paper is

10

0

0.5

1.0

1.5

2.0

2.5 x

Fig. 17.5

3.0

3.5

4.0

4.5

128

Basic Engineering Mathematics

Gradient of straight line, lg b = AB/BC. Direct measurement (say in centimetres) is not made with log-linear graph paper since the vertical scale is logarithmic and the horizontal scale is linear. Hence lg 1000 − lg 100 3−2 AB = = BC 3.82 − 2.02 1.80 1 = = 0.5556 1.80

17.4 Graphs of the form y = aekx Taking logarithms to a base of e of both sides of y = aekx gives: ln y = ln(aekx ) = ln a + ln ekx = ln a + kx ln e ln y = kx + ln a (since ln e = 1)

i.e.

which compares with Y = mX + c

Hence b = antilog 0.5556(=100.5556 ) = 3.6, correct to 2 significant figures. Point A has coordinates (3.82, 1000). Substituting these values into y = abx gives: 1000 = a(3.6)3.82 , i.e.

Thus, by plotting ln y vertically against x horizontally, a straight line results, i.e. the equation y = aekx is reduced to linear form. In this case,. graph paper having a linear horizontal scale and a logarithmic vertical scale may be used. Problem 5. The data given below is believed to be realted by a law of the form y = aekx , where a and b are constants. Verify that the law is true and determine approximate values of a and b. Also determine the value of y when x is 3.8 and the value of x when y is 85.

1000 a= (3.6)3.82 = 7.5, correct to 2 significant figures.

x y

Hence the law of the graph is: y = 7.5(3.6)x

−1.2 9.3

0.38 22.2

1.2 34.8

2.5 71.2

3.4 117

4.2 181

5.3 332

(i) When x = 2.5, y = 7.5(3.6)2.5 = 184 (ii) When y = 1200, 1200 = 7.5(3.6)x , hence (3.6)x =

Since y = aekx then ln y = kx + ln a (from above), which is of the form Y = mX + c, showing that to produce a straight line graph ln y is plotted vertically against x horizontally. The value of y ranges from 9.3 to 332 hence ‘log 3 cycle × linear’ graph paper is used. The ploted co-ordinates are shown in Fig. 17.6 and since a straight line passes through the points the law y = aekx is verified. Gradient of straight line,

1200 = 160 7.5

Taking logarithms gives: xlg 3.6 = lg 160 x=

i.e.

lg 160 2.2041 = lg 3.6 0.5563

AB ln 100 − ln 10 2.3026 = = BC 3.12 − (−1.08) 4.20 = 0.55, correct to 2 significant figures.

k =

= 3.96

Since ln y = kx + ln a, when x = 0, ln y = ln a, i.e. y = a.

Now try the following exercise Exercise 63 Further problem on graphs of the form y = abx (Answers on page 277) 1. Experimental values of p and corresponding values of q are shown below.

The vertical axis intercept value at x = 0 is 18, hence a = 18. The law of the graph is thus: y = 18e0.55x When x is 3.8, y = 18e0.55(3.8) = 18e2.09 = 18(8.0849) = 146

p

−13.2 −27.9 −62.2 −383.2 −1581 −2931

q

0.30

0.75

1.23

2.32

3.17

When y is 85, 85 = 18e0.55x

3.54 Hence, e0.55x =

Show that the law relating p and q is p = abq , where a and b are constants. Determine (i) values of a and b, and state the law, (ii) the value of p when q is 2.0, and (iii) the value of q when p is −2000.

85 = 4.7222 18

and 0.55x = ln 4.7222 = 1.5523. Hence x =

1.5523 = 2.82 0.55

Graphs with logarithmic scales

129

1000

1000 y

t

y ⫽ aekx

100

v ⫽ VeT (36.5, 100)

100

A

Voltage, v volts

A

10

0 ⫺2

B

C

⫺1

0

1

2

3

10

4

5

6

x

1

B

0

10

20

Fig. 17.6

30

C

40 50 Time, t ms

60

70

80

90

Fig. 17.7 Problem 6. The voltage, v volts, across an inductor is believed to be related to time, t ms, by the law v = Vet/T , where V and T are constants. Experimental results obtained are: v volts t ms

883 10.4

347 21.6

90 37.8

55.5 43.6

18.6 56.7

Since the straight line does not cross the vertical axis at t = 0 in Fig. 17.7, the value of V is determined by selecting any point, say A, having co-ordinates (36.5, 100) and substituting these values into v = Vet/T . Thus

5.2 72.0 i.e

Show that the law relating voltage and time is as stated and determine the approximate values of V and T . Find also the value of voltage after 25 ms and the time when the voltage is 30.0 V 1 t + ln V T which is of the form Y = mX + c. Using ‘log 3 cycle × linear’ graph paper, the points are plotted as shown in Fig. 17.7. Since the points are joined by a straight line the law v = Vet/T is verified. Gradient of straight line, Since v = Vet/T then ln v =

1 AB ln 100 − ln 10 2.3026 = = = T BC 36.5 − 64.2 −27.7 Hence T =

−27.7 = −12.0, correct to 3 significant figures. 2.3026

100 = Ve36.5/−12.0 100 V = −36.5/12.0 = 2090 volts. e correct to 3 significant figures.

Hence the law of graph is: υ = 2090e−t/12.0 When time t = 25 ms, voltage υ = 2090e−25/12.0 = 260 V. When the voltage is 30.0 volts, 30.0 = 2090e−t/12.0 , hence e−t/12.0 =

30.0 2090

and

et/12.0 =

2090 = 69.67 30.0

Taking Napierian logarithms gives: t = ln 69.67 = 4.2438 12.0 from which, time t = (12.0)(4.2438) = 50.9 ms.

130

Basic Engineering Mathematics

Now try the following exercise Exercise 64 Further problems on reducing exponential laws to linear form (Answers on page 277) 1. Atmospheric pressure p is measured at varying altitudes h and the results are as shown below: Altitude, h m pressure, p cm

500 1500 3000 5000 8000 73.39 68.42 61.60 53.56 43.41

Show that the quantities are related by the law p = aekh , where a and k are constants. Determine the values of a and k and state the law. Find also the atmospheric pressure at 10 000 m. 2. At particular times, t minutes, measurements are made of the temperature, θ ◦ C, of a cooling liquid and the following results are obtained: Temperature θ ◦C Time t minutes

92.2

55.9

33.9

20.6

12.5

10

20

30

40

50

Prove that the quantities follow a law of the form θ = θ0 ekt , where θ0 and k are constants, and determine the approximate value of θ0 and k.

18 Geometry and triangles

18.1 Angular measurement Geometry is a part of mathematics in which the properties of points, lines, surfaces and solids are investigated. An angle is the amount of rotation between two straight lines. Angles may be measured in either degrees or radians (see Section 23.3). 1 1 revolution = 360 degrees, thus 1 degree = th of one revo360 1 1 lution. Also 1 minute = th of a degree and 1 second = th 60 60 of a minute. 1 minute is written as 1 and 1 second is written as 1

◦ Thus 1 = 60 and 1 = 60

13 − 47 cannot be done. Hence 1◦ or 60 is ‘borrowed’ from the degrees column, which leaves 27◦ in that column. Now (60 + 13 ) − 47 = 26 , which is placed in the minutes column. 27◦ − 15◦ = 12◦ , which is placed in the degrees column. Thus 28◦ 13 − 15◦ 47 = 12◦ 26 Problem 3. Determine (a) 13◦ 42 51

+ 48◦ 22 17

(b) 37◦ 12 8

− 21◦ 17 25

13◦ 42 51

48◦ 22 17

(a)

Problem 1. Add 14◦ 53 and 37◦ 19

62◦ 5 8

Adding:

1◦ 1 14◦ 53 37◦ 19 52◦ 12

36◦ 11 3 7 ◦ 1 2 8

◦ 21 17 25

(b)

1◦ 53 + 19 = 72 . Since 60 = 1◦ , 72 = 1◦ 12 . Thus the 12 is placed in the minutes column and 1◦ is carried in the degrees column. Then 14◦ + 37◦ + 1◦ (carried) = 52◦ Thus

15◦ 54 43

Subtracting:

Problem 4. Convert (a) 24◦ 42 (b) 78◦ 15 26

to degrees and decimals of a degree.

14◦ 53 + 37◦ 19 = 52◦ 12

Problem 2. Subtract 15◦ 47 from 28◦ 13 27◦ 2 8 ◦ 13 15◦ 47 12◦ 26

(a) Since 1 minute =

42 =

42 60

◦

1 th of a degree, 60 = 0.70◦

Hence 24◦ 42 = 24.70◦

132

Basic Engineering Mathematics

(b) Since 1 second =

26

=

26 60

1 th of a minute, 60

(iii) Any angle between 90◦ and 180◦ is called an obtuse angle.

= 0.4333

(iv) Any angle greater than 180◦ and less than 360◦ is called a reflex angle. (b) (i) An angle of 180◦ lies on a straight line.

Hence 78 15 26 = 78 15.43˙ ◦

15.4333 =

◦

15.43˙ 60

◦

(ii) If two angles add up to 90◦ they are called complementary angles.

= 0.2572◦ ,

(iii) If two angles add up to 180◦ they are called supplementary angles.

correct to 4 decimal places. ◦

◦

Hence 78 15 26 = 78.26 , correct to 4 significant places. Problem 5. Convert 45.371◦ into degrees, minutes and seconds.

(iv) Parallel lines are straight lines which are in the same plane and never meet. (Such lines are denoted by arrows, as in Fig. 18.1). (v) A straight line which crosses two parallel lines is called a transversal (see MN in Fig. 18.1).

Since 1◦ = 60 , 0.371◦ = (0.371 × 60) = 22.26

N

Since: 1 = 60

, 0.26 = (0.26×60)

= 15.6

= 16

to the nearest second. Hence

◦

◦

a

d

P

c

b

Q

45.371 = 45 22 16

h e g f

R

Now try the following exercise Exercise 65

Further problems on angular measurement (Answers on page 277)

S

M

Fig. 18.1

1. Add together the following angles: (c) With reference to Fig. 18.1:

(a) 32◦ 19 and 49◦ 52 ◦

◦

◦

(b) 29 42 , 56 37 and 63 54

(i) a = c, b = d, e = g and f = h. Such pairs of angles are called vertically opposite angles.

(c) 21◦ 33 27

and 78◦ 42 36

(ii) a = e, b = f , c = g and d = h. Such pairs of angles are called corresponding angles.

(d) 48◦ 11 19

, 31◦ 41 27

and 9◦ 9 37

2. Determine: (a) 17◦ − 9◦ 49

(b) 43◦ 37 − 15◦ 49

(c) 78◦ 29 41

− 59◦ 41 52

(d) 114◦ − 47◦ 52 37

3. Convert the following angles to degrees and decimals of a degree, correct to 3 decimal places: (a) 15◦ 11

(b) 29◦ 53

(c) 49◦ 42 17

(b) 36.48◦

(c) 55.724◦

(iv) b + e = 180◦ and c + h = 180◦ . Such pairs of angles are called interior angles.

(d) 135◦ 7 19

4. Convert the following angles into degrees, minutes and seconds: (a) 25.4◦

(iii) c = e and b = h. Such pairs of angles are called alternate angles.

(d) 231.025◦

18.2 Types and properties of angles (a) (i) Any angle between 0◦ and 90◦ is called an acute angle. (ii) An angle equal to 90◦ is called a right angle.

Problem 6. State the general name given to the following angles: (a) 159◦ (b) 63◦ (c) 90◦ (d) 227◦

(a) 159◦ lies between 90◦ and 180◦ and is therefore called an obtuse angle. (b) 63◦ lies between 0◦ and 90◦ and is therefore called an acute angle. (c) 90◦ is called a right angle.

Geometry and triangles

(d) 227◦ is greater than 180◦ and less than 360◦ and is therefore called a reflex angle.

Problem 11. A

Problem 7. Find the angles complementary to (a) 41◦ (b) 58◦ 39

F

(a) The complement of 41◦ is (90◦ − 41◦ ), i.e. 49◦ ◦

◦

◦

C ◦

(b) The complement of 58 39 is (90 − 58 39 ), i.e. 31 21

133

Determine the value of angle θ in Fig. 18.4. B

23°37′ u

G

E 35°49′

D

Fig. 18.4

(a) The supplement of 27◦ is (180◦ − 27◦ ), i.e. 153◦

Let a straight line FG be drawn through E such that FG is parallel to AB and CD. ∠BAE = ∠AEF (alternate angles between parallel lines AB and FG), hence ∠AEF = 23◦ 37 . ∠ECD = ∠FEC (alternate angles between parallel lines FG and CD), hence ∠FEC = 35◦ 49

(b) The supplement of 111◦ 11 is (180◦ − 111◦ 11 ), i.e. 68◦ 49

Angle θ = ∠AEF + ∠FEC = 23◦ 37 + 35◦ 49 = 59◦ 26

Problem 8. Find the angles supplementary to (a) 27◦ (b) 111◦ 11

Problem 9. Two straight lines AB and CD intersect at 0. If ∠AOC is 43◦ , find ∠AOD, ∠DOB and ∠BOC.

Problem 12.

Determine angles c and d in Fig. 18.5.

From Fig. 18.2, ∠AOD is supplementary to ∠AOC. Hence ∠AOD = 180◦ − 43◦ = 137◦ . When two straight lines intersect the vertically opposite angles are equal. Hence ∠DOB = 43◦ and ∠BOC = 137◦

d

46° a

b c

D

A

Fig. 18.5 43°

0 B

C

b = 46◦ (corresponding angles between parallel lines). Also b + c + 90◦ = 180◦ (angles on a straight line). Hence 46◦ + c + 90◦ = 180◦ , from which c = 44◦ .

Fig. 18.2

b and d are supplementary, hence d = 180◦ − 46◦ = 134◦ . Alternatively, 90◦ + c = d (vertically opposite angles). Problem 10. Determine angle β in Fig. 18.3. Now try the following exercise

133° a b

Exercise 66

Further problems on types and properties of angles (Answers on page 277)

1. State the general name given to the (a) 63◦

(b) 147◦

(c) 250◦

2. Determine the angles complementary to the following: Fig. 18.3 α = 180◦ − 133◦ = 47◦ (i.e. supplementary angles). α = β = 47◦ (corresponding angles between parallel lines).

(a) 69◦

(b) 27◦ 37

(c) 41◦ 3 43

3. Determine the angles supplementary to (a) 78◦

(b) 15◦

(c) 169◦ 41 11

134

Basic Engineering Mathematics

4. With reference to Fig. 18.6, what is the name given to the line XY . Give examples of each of the following: (a) vertically opposite angles. (b) supplementary angles. (c) corresponding angles. (d) alternate angles.

18.3 Properties of triangles A triangle is a figure enclosed by three straight lines. The sum of the three angles of a triangle is equal to 180◦ . Types of triangles: (i) An acute-angled triangle is one in which all the angles are acute, i.e. all the angles are less than 90◦ . (ii) A right-angled triangle is one which contains a right angle.

y 6 5 8 7 x

(iii) An obtuse-angled triangle is one which contains an obtuse angle, i.e. one angle which lies between 90◦ and 180◦ . (iv) An equilateral triangle is one in which all the sides and all the angles are equal (i.e. each 60◦ ).

2 1 43

(v) An isosceles triangle is one in which two angles and two sides are equal. (vi) A scalene triangle is one with unequal angles and therefore unequal sides.

Fig. 18.6

With reference to Fig. 18.10: 5. In Fig. 18.7, find angle α. A 137°29′ b

c

a 16°49′ u

Fig. 18.7

C

B a

6. In Fig. 18.8, find angles a, b and c.

Fig. 18.10 (i) Angles A, B and C are called interior angles of the triangle.

29°

c

(ii) Angle θ is called an exterior angle of the triangle and is equal to the sum of the two opposite interior angles, i.e. θ = A + C

a

(iii) a + b + c is called the perimeter of the triangle.

69° b

Problem 13. Fig. 18.11.

Name the types of triangles shown in

2.6

Fig. 18.8 2

2.1

2

39°

2.8

7. Find angle β in Fig. 18.9.

(a)

(c)

(b)

2

51°

133° 2.5

98° (d)

Fig. 18.9

2.1

107°

b

Fig. 18.11

(e)

2.5

Geometry and triangles

(a) Equilateral triangle.

Problem 16.

135

Find angles a, b, c, d and e in Fig. 18.14.

(b) Acute-angled scalene triangle. (c) Right-angled triangle. a 55° b

(d) Obtuse-angled scalene triangle. (e) Isosceles triangle.

e

62° d

c

Problem 14. Determine the value of θ and α in Fig. 18.12. A 62°

Fig. 18.14

D

B

u

C

15° E a

a = 62◦ and c = 55◦ (alternate angles between parallel lines) 55◦ + b + 62◦ = 180◦ (angles in a triangle add up to 180◦ ), hence b = 180◦ − 55◦ − 62◦ = 63◦ b = d = 63◦ (alternate angles between parallel lines). e + 55◦ + 63◦ = 180◦ (angles in a triangle add up to 180◦ ), hence e = 180◦ − 55◦ − 63◦ = 62◦

Fig. 18.12 In triangle ABC, ∠A + ∠B + ∠C = 180◦ (angles in a triangle add up to 180◦ ), hence ∠C = 180◦ − 90◦ − 62◦ = 28◦ . Thus ∠DCE = 28◦ (vertically opposite angles). θ = ∠DCE + ∠DEC (exterior angle of a triangle is equal to the sum of the two opposite interior angles). Hence ∠θ = 28◦ + 15◦ = 43◦ ∠α and ∠DEC are supplementary, thus ◦

◦

α = 180 − 15 = 165

[Check: e = a = 62◦ (corresponding angles between parallel lines)].

Now try the following exercise Exercise 67

◦

Problem 15. ABC is an isosceles triangle in which the unequal angle BAC is 56◦ . AB is extended to D as shown in Fig. 18.13. Determine the angle DBC.

Further problems on properties of triangles (Answers on page 278)

1. In Fig. 18.15, (i) and (ii), find angles w, x, y and z. What is the name given to the types of triangle shown in (i) and (ii)? v

A 2 cm 70°

2 cm

56° 110° x

y 110°

z

(i) B

(ii)

C

Fig. 18.15 D

2. Find the values of angles a to g in Fig. 18.16 (i) and (ii).

Fig. 18.13 68°

Since the three interior angles of a triangle add up to 180◦ then 56◦ + ∠B + ∠C = 180◦ , i.e. ∠B + ∠C = 180◦ − 56◦ = 124◦ . 124◦ = 62◦ . 2 ∠DBC = ∠A + ∠C (exterior angle equals sum of two interior opposite angles), i.e. ∠DBC = 56◦ + 62◦ = 118◦ [Alternatively, ∠DBC + ∠ABC = 180◦ (i.e. supplementary angles)].

Triangle ABC is isosceles hence ∠B = ∠C =

c d

56°29′

e g

b 14°41′ a (i)

Fig. 18.16

f 131° (ii)

136

Basic Engineering Mathematics

3. Find the unknown angles a to k in Fig. 18.17.

Problem 17. State which of the pairs of triangles shown in Fig. 18.19 are congruent and name their sequence.

f

G

C E

L

D

J

B

K

g

125° e 22°

b

j

d

a c

h

H

(a) A

i

T D

S

k 99°

F

(b)

I F

(d)

M

V

U

E C

B

(e) O

N P

Fig. 18.17

Q

(c)

4. Triangle ABC has a right angle at B and ∠BAC is 34 . BC is produced to D. If the bisectors of ∠ABC and ∠ACD meet at E, determine ∠BEC. 5. If in Fig. 18.18, triangle BCD is equilateral, find the interior angles of triangle ABE. A C

W

R

◦

A

X

Fig. 18.19 (a) Congruent ABC, FDE (Angle, side, angle, i.e. ASA). (b) Congruent GIH , JLK (Side, angle, side, i.e. SAS). (c) Congruent MNO, RQP (Right-angle, hypotenuse, side, i.e. RHS). (d) Not necessarily congruent. It is not indicated that any side coincides.

B

(e) Congruent ABC, FED (Side, side, side, i.e. SSS). E

97°

D

Fig. 18.18

Problem 18. In Fig. 18.20, triangle PQR is isosceles with Z the mid-point of PQ. Prove that triangle PXZ and QYZ are congruent, and that triangles RXZ and RYZ are congruent. Determine the values of angles RPZ and RXZ. R X

18.4 Congruent triangles P

Two triangles are said to be congruent if they are equal in all respects, i.e. three angles and three sides in one triangle are equal to three angles and three sides in the other triangle. Two triangles are congruent if: (i) the three sides of one are equal to the three sides of the other (SSS),

Y

67° 28°

28° Z

Q

Fig. 18.20 Since triangle PQR is isosceles PR = RQ and thus ∠QPR = ∠RQP

(ii) they have two sides of the one equal to two sides of the other, and if the angles included by these sides are equal (SAS),

∠RXZ = ∠QPR + 28◦ and ∠RYZ = ∠RQP + 28◦ (exterior angles of a triangle equal the sum of the two interior opposite angles). Hence ∠RXZ = ∠RYZ.

(iii) two angles of the one are equal to two angles of the other and any side of the first is equal to the corresponding side of the other (ASA), or

∠PXZ = 180◦ − ∠RXZ ∠PXZ = ∠QYZ.

(iv) their hypotenuses are equal and if one other side of one is equal to the corresponding side of the other (RHS).

and

∠QYZ = 180◦ − ∠RYZ.

Thus

Triangles PXZ and QYZ are congruent since ∠XPZ = ∠YQZ, PZ = ZQ and ∠XZP = ∠YZQ (ASA)

Geometry and triangles

Hence XZ = YZ Triangles PRZ and QRZ are congruent since PR = RQ, ∠RPZ = ∠RQZ and PZ = ZQ (SAS). Hence ∠RZX = ∠RZY Triangles RXZ and RYZ are congruent since ∠RXZ = ∠RYZ, XZ = YZ and ∠RZX = ∠RZY (ASA). ∠QRZ = 67◦ and thus ∠PRQ = 67◦ + 67◦ = 134◦ . Hence

137

to Fig. 18.22: Triangles ABC and PQR are similar and the corresponding sides are in proportion to each other, i.e. p q r = = a b c A

180◦ − 134◦ = 23◦ 2 ∠RXZ = 23◦ + 28◦ = 51◦ (external angle of a triangle equals the sum of the two interior opposite angles).

P

57°

∠RPZ = ∠RQZ =

c

b r 57° q

65°

58°

B

a

C Q

65° 58° p R

Fig. 18.22 Now try the following exercise Exercise 68 Further problems on congruent triangles (Answers on page 278)

Problem 19.

In Fig. 18.23, find the length of side a. A

1. State which of the pairs of triangles in Fig. 18.21 are congruent and name their sequence. A

50° D

c ⫽ 12.0 cm

L

f ⫽ 5.0 cm 50°

F E

K

G

M

I C

Fig. 18.23 P

J

(b)

(c) V

In triangle ABC, 50◦ + 70◦ + ∠C = 180◦ , from which ∠C = 60◦ In triangle DEF, ∠E = 180◦ − 50◦ − 60◦ = 70◦ . Hence triangles ABC and DEF are similar, since their angles are the same. Since corresponding sides are in proportion to each other then:

U R

Q

C

N

D (a)

a

O

H

B

60° E F d ⫽ 4.42 cm

70°

B

a c = d f

W

S

Y

(e)

X

Hence

a=

i.e.

a 12.0 = 4.42 5.0

12.0 (4.42) = 10.61 cm 5.0

(d) T Z

Problem 20. r and p.

In Fig. 18.24, find the dimensions marked

Fig. 18.21

q ⫽ 6.82 cm

Z

p 35°

18.5 Similar triangles Two triangles are said to be similar if the angles of one triangle are equal to the angles of the other triangle. With reference

x ⫽ 7.44 cm

.97

Q

12

P

cm

55° r

z⫽

2. In a triangle ABC, AB = BC and D and E are points on AB and BC, respectively, such that AD = CE. Show that triangles AEB and CDB are congruent.

Y

R

Fig. 18.24

y⫽

X

m

63 c

10.

138

Basic Engineering Mathematics

In triangle PQR, ∠Q = 180◦ − 90◦ − 35◦ = 55◦ ◦

◦

◦

◦

In triangle XYZ, ∠X = 180 − 90 − 55 = 35

Hence triangles PQR and ZYX are similar since their angles are the same. The triangles may be redrawn as shown in Fig. 18.25. Y

55° p

r

35° q ⫽ 6.82 cm

P

= 7.2 cm

55°

x ⫽ 7.44 cm

Q

z⫽

12

.97

Hence

from which, By proportion:

Hence

9 BD 9 = , from which BD = 10 15 10 15

Also,

cm

= 6 cm 35°

R Z

y ⫽ 10.63 cm

X

Fig. 18.25 p r q = = z x y

By proportion:

Also ∠C is common to triangles CBD and CAE. Since the angles in triangle CBD are the same as in triangle CAE the triangles are similar. Hence, by proportion: CB CD BD = = CA CE AE 9 9 CD i.e. , from which CD = 12 = 12 15 6+9

p r 6.82 = = z 7.44 10.63 6.82 r = 7.44 = 4.77 cm 10.63 p q = z y

p 6.82 = 12.97 10.63 6.82 p = 12.97 = 8.32 cm 10.63

Problem 22. A rectangular shed 2 m wide and 3 m high stands against a perpendicular building of height 5.5 m. A ladder is used to gain access to the roof of the building. Determine the minimum distance between the bottom of the ladder and the shed.

A side view is shown in Fig. 18.27, where AF is the minimum length of ladder. Since BD and CF are parallel, ∠ADB = ∠DFE (corresponding angles between parallel lines). Hence triangles BAD and EDF are similar since their angles are the same. AB = AC − BC = AC − DE = 5.5 − 3 = 2.5 m.

i.e.

By proportion:

Hence

AB BD 2.5 2 = i.e. = DE EF 3 EF 3 EF = 2 = 2.4 m = minimum 2.5 distance from bottom of ladder to the shed

Problem 21. In Fig. 18.26, show that triangles CBD and CAE are similar and hence find the length of CD and BD. A A 6 B

E

2m C

D

B

D

9

10

3m

12 F

Fig. 18.26

Since BD is parallel to AE then ∠CBD = ∠CAE and ∠CDB = ∠CEA (corresponding angles between parallel lines).

Fig. 18.27

E

5.5 m

Shed C

Geometry and triangles

139

Now try the following exercise C

Exercise 69 Further problems on similar triangles (Answers on page 278)

D

B

1. In Fig. 18.28, find the lengths x and y.

A 14.58 mm

F

E

Fig. 18.30

25.69 mm 111°

18.6 Construction of triangles x y

4.74 mm

32°

37°

32° 7.36 mm

To construct any triangle the following drawing instruments are needed: (i) ruler and/or straight edge, (ii) compass, (iii) protractor, (iv) pencil. For actual constructions, see Problems 23 to 26 which follow.

Fig. 18.28 Problem 23. Construct a triangle whose sides are 6 cm, 5 cm and 3 cm. 2. PQR is an equilateral triangle of side 4 cm. When PQ and PR are produced to S and T , respectively, ST is found to be parallel with QR. If PS is 9 cm, find the length of ST . X is a point on ST between S and T such that the line PX is the bisector of ∠SPT . Find the length of PX .

With reference to Fig. 18.31: D G

C

3. In Fig. 18.29, find (a) the length of BC when AB = 6 cm, DE = 8 cm and DC = 3 cm, (b) the length of DE when EC = 2 cm, AC = 5 cm and AB = 10 cm.

F

A

6 cm

E B

Fig. 18.31 A

B

(i) Draw a straight line of any length, and with a pair of compasses, mark out 6 cm length and label it AB. (ii) Set compass to 5 cm and with centre at A describe arc DE. (iii) Set compass to 3 cm and with centre at B describe arc FG. (iv) The intersection of the two curves at C is the vertex of the required triangle. Join AC and BC by straight lines.

C

D

E

Fig. 18.29

4. In Fig. 18.30, AF = 8 m, AB = 5 m and BC = 3 m. Find the length of BD.

It may be proved by measurement that the ratio of the angles of a triangle is not equal to the ratio of the sides (i.e. in this problem, the angle opposite the 3 cm side is not equal to half the angle opposite the 6 cm side).

Problem 24. Construct a triangle ABC such that a = 6 cm, b = 3 cm and ∠C = 60◦ .

140

Basic Engineering Mathematics

V Z P U

A b ⫽ 3 cm

C Q

R

60° a ⫽ 6 cm

B

S

C

B

Fig. 18.32

A

X

A′

Y

Fig. 18.34

With reference to Fig. 18.32:

With reference to Fig. 18.34:

(i) Draw a line BC, 6 cm long.

(i) Draw a straight line 5 cm long and label it XY . ◦

(ii) Using a protractor centred at C make an angle of 60 to BC. (iii) From C measure a length of 3 cm and label A. (iv) Join B to A by a straight line. Problem 25. Construct a triangle PQR given that QR = 5 cm, ∠Q = 70◦ and ∠R = 44◦ .

(iii) The hypotenuse is always opposite the right angle. Thus YZ is opposite ∠X . Using a compass centred at Y and set to 6.5 cm, describe the arc UV .

With reference to Fig. 18.33: Q′

R′

(ii) Produce XY any distance to B. With compass centred at X make an arc at A and A . (The length XA and XA is arbitrary.) With compass centred at A draw the arc PQ. With the same compass setting and centred at A , draw the arc RS. Join the intersection of the arcs, C, to X , and a right angle to XY is produced at X . (Alternatively, a protractor can be used to construct a 90◦ angle).

(iv) The intersection of the arc UV with XC produced, forms the vertex Z of the required triangle. Join YZ by a straight line.

P

Now try the following exercise Exercise 70 70° Q

44° 5 cm

R

Fig. 18.33

Further problems on the construction of triangles (Answers on page 278)

In problems 1 to 5, construct the triangles ABC for the given sides/angles. 1. a = 8 cm, b = 6 cm and c = 5 cm 2. a = 40 mm, b = 60 mm and C = 60◦

(i) Draw a straight line 5 cm long and label it QR.

3. a = 6 cm, C = 45◦ and B = 75◦

(ii) Use a protractor centred at Q and make an angle of 70◦ . Draw QQ .

4. c = 4 cm, A = 130◦ and C = 15◦

(iii) Use a protractor centred at R and make an angle of 44◦ . Draw RR . (iv) The intersection of QQ and RR forms the vertex P of the triangle. Problem 26. Construct a triangle XYZ given that XY = 5 cm, the hypotenuse YZ = 6.5 cm and ∠X = 90◦ .

5. a = 90 mm, B = 90◦ , hypotenuse = 105 mm

Geometry and triangles

Assignment 8

8. In Fig. A8.1, determine angles x, y and z

This assignment covers the material contained in chapters 16 to 18. The marks for each question are shown in brackets at the end of each question. y

1. In the following equations, x and y are two related variables and k and t are constants. For the stated equations to be verified it is necessary to plot graphs of the variables in modified form. State for each (a) what should be plotted on the horizontal axis, (b) what should be plotted on the vertical axis, (c) the gradient, and (d) the vertical axis intercept. (i) y −

k =t x

(ii)

y = xt k

(8)

141

(3)

59° 37° z x

Fig. A8.1 9. In Fig. A8.2, determine angles a to e

(5)

c e

2. The following experimental values of x and y are believed to be related by the law y = ax2 + b, where a and b are constants. By plotting a suitable graph verify this law and find the approximate values of a and b.

b

a

d

125°

60°

x

2.5

4.2

6.0

8.4

9.8

11.4

y

15.4

32.5

60.2

111.8

150.1

200.9 (8)

3. State the minimum number of cycles on logarithmic graph paper needed to plot a set of values ranging from 0.065 to 480. (2)

Fig. A8.2 10. In Fig. A8.3, determine the length of AC

(4)

B

4. Determine the law of the form y = aekx which relates the following values: 10 m

y x

0.0306 0.285 0.841 −4.0

5.3

9.8

5.21 173.2 1181 17.4

32.0

D

40.0

3m

A C

(8) 5. Evaluate: 29◦ 17 + 75◦ 51 − 47◦ 49

(3)

8m

6. Convert 47.319◦ to degrees, minutes and seconds (2) Fig. A8.3

7. State the angle (a) supplementary to 49◦ (b) complementary to 49◦

11. Construct a triangle PQR given PQ = 5 cm, (2)

∠QPR = 120◦ and ∠PRQ = 35◦

(5)

19 Introduction to trigonometry

By Pythagoras’ theorem, a2 = b2 + c2

19.1 Trigonometry Trigonometry is the branch of mathematics which deals with the measurement of sides and angles of triangles, and their relationship with each other. There are many applications in engineering where a knowledge of trigonometry is needed.

19.2 The theorem of Pythagoras With reference to Fig. 19.1, the side opposite the right angle (i.e. side b) is called the hypotenuse. The theorem of Pythagoras states: ‘In any right-angled triangle, the square on the hypotenuse is equal to the sum of the squares on the other two sides.’ Hence b2 = a2 + c2

a2 = 42 + 32 = 16 + 9 = 25 √ Hence a = 25 = ±5 (−5 has no meaning in this context and is thus ignored)

i.e.

bc = 5 cm

Thus

Problem 2.

In Fig. 19.3, find the length of EF D f ⫽ 5 cm E

e ⫽ 13 cm d

F

Fig. 19.3

A

By Pythagoras’ theorem: e2 = d 2 + f 2 b

c B

Hence

169 = d 2 + 25

C

a

Fig. 19.1 Thus Problem 1.

In Fig. 19.2, find the length of BC

132 = d 2 + 52

i.e.

d 2 = 169 − 25 = 144 √ d = 144 = 12 cm EF = 12 cm

C

b ⫽ 4 cm

A

Fig. 19.2

Problem 3. Two aircraft leave an airfield at the same time. One travels due north at an average speed of 300 km/h and the other due west at an average speed of 220 km/h. Calculate their distance apart after 4 hours.

a

c ⫽ 3 cm

B

After 4 hours, the first aircraft has travelled 4 × 300 = 1200 km, due north, and the second aircraft has travelled 4 × 220 = 880 km due west, as shown in Fig. 19.4. Distance apart after 4 hours = BC

Introduction to trigonometry

From Pythagoras’ theorem:

and

2

8. Figure 19.5 shows a bolt rounded off at one end. Determine the dimension h.

2

= 1 440 000 + 774 400 BC = (2 214 400)

r ⫽ 16 mm

BC = 1200 + 880 2

h

N W

143

E S

R ⫽ 45 mm

B

1200 km

C

A 880 km

Fig. 19.4 Hence distance apart after 4 hours = 1488 km. Fig. 19.5 Now try the following exercise Exercise 71 Further problems on the theorem of Pythagoras (Answers on page 278)

9. Figure 19.6 shows a cross-section of a component that is to be made from a round bar. If the diameter of the bar is 74 mm, calculate the dimension x.

1. In a triangle ABC, ∠B is a right angle, AB = 6.92 and BC = 8.78 cm. Find the length of the hypotenuse.

x

2. In a triangle CDE, D = 90◦ , CD = 14.83 mm and CE = 28.31 mm. Determine the length of DE. 3. Show that if a triangle has sides of 8, 15 and 17 cm it is right angled.

m

4m

f7

72 mm

4. Triangle PQR is isosceles, Q being a right angle. If the hypotenuse is 38.47 cm find (a) the lengths of sides PQ and QR, and (b) the value of ∠QPR. 5. A man cycles 24 km due south and then 20 km due east. Another man, starting at the same time as the first man, cycles 32 km due east and then 7 km due south. Find the distance between the two men. 6. A ladder 3.5 m long is placed against a perpendicular wall with its foot 1.0 m from the wall. How far up the wall (to the nearest centimetre) does the ladder reach? If the foot of the ladder is now moved 30 cm further away from the wall, how far does the top of the ladder fall? 7. Two ships leave a port at the same time. One travels due west at 18.4 km/h and the other due south at 27.6 km/h. Calculate how far apart the two ships are after 4 hours.

Fig. 19.6

19.3 Trigonometric ratios of acute angles (a) With reference to the right-angled triangle shown in Fig. 19.7: (i) sine θ =

opposite side , hypotenuse

(ii) cosine θ =

adjacent side , hypotenuse

(iii) tangent θ =

opposite side , adjacent side

i.e. sin θ =

b c

i.e. cos θ =

a c

i.e. tan θ =

b a

144

Basic Engineering Mathematics

Problem 6.

c

b

If cos X =

and tan X .

9 determine the value of sin X 41

u a

Figure 19.10 shows a right-angled triangle XYZ.

Fig. 19.7 Z

Problem 4.

From Fig. 19.8, find sin D, cos D and tan F F 41 17

D

8

X

E

Fig. 19.10

Fig. 19.8

Since cos X =

By Pythagoras’ theorem, 172 = 82 + EF 2 √ from which, EF = 172 − 82 = 15 sin D =

15 EF = DF 17

or 0.8824

cos D =

8 DE = DF 17

or 0.4706

tan F =

DE 8 = EF 15

or 0.5333

A

u 12

Thus sin X =

40 41

and tan X =

5

Problem 7. Point A lies at co-ordinate (2,3) and point B at (8,7). Determine (a) the distance AB, (b) the gradient of the straight line AB, and (c) the angle AB makes with the horizontal.

In Fig. 19.11(b), the horizontal and vertical lines AC and BC are constructed.

C f (x)

Fig. 19.9

f (x)

8 7 6

sin θ =

5 opposite side = = 0.3846 hypotenuse 13

adjacent side 12 cos θ = = = 0.9231 hypotenuse 13 tan θ =

40 4 =4 9 9

(a) Points A and B are shown in Fig. 19.11(a).

B 13

9 , then XY = 9 units and XZ = 41 units. 41

Using√ Pythagoras’ theorem: 412 = 92 + YZ 2 from which YZ = 412 − 92 = 40 units.

Problem 5. Determine the values of sin θ, cos θ and tan θ for the right-angled triangle ABC shown in Fig. 19.9.

By definition:

Y

9

opposite side 5 = = 0.4167 adjacent side 12

4 3 2

8 B

B 6 4

A

0

2 2

4

6

(a)

Fig. 19.11

8

0

u

C

A

2

4

6

(b)

8

Introduction to trigonometry

Since ABC is a right-angled triangle, and AC = (8 − 2) = 6 and BC = (7 − 3) = 4, then by Pythagoras’ theorem

7. Point P lies at co-ordinate (−3,1) and point Q at (5,−4). Determine

AB2 = AC 2 + BC 2 = 62 + 42 and AB = places.

145

(a) the distance PQ, (b) the gradient of the straight line PQ and,

√ √ 62 + 42 = 52 = 7.211, correct to 3 decimal

(c) the angle PQ makes with the horizontal.

(b) The gradient of AB is given by tan θ,

19.4 Solution of right-angled triangles i.e.

BC 4 2 = = gradient = tan θ = AC 6 3

(c) The angle AB makes with the horizontal is given by 2 tan−1 = 33.69◦ 3

To ‘solve a right-angled triangle’ means ‘to find the unknown sides and angles’. This is achieved by using (i) the theorem of Pythagoras, and/or (ii) trigonometric ratios. This is demonstrated in the following problems.

Problem 8. Sketch a right-angled triangle ABC such that B = 90◦ , AB = 5 cm and BC = 12 cm. Determine the length of AC and hence evaluate sin A, cos C and tan A.

Now try the following exercise Exercise 72 Further problems on trigonometric ratios of actue angles (Answers on page 278) ◦

1. Sketch a triangle XYZ such that ∠Y = 90 , XY = 9 cm and YZ = 40 cm. Determine sin Z, cos Z,tan X and cos X . 2. In triangle ABC shown in Fig. 19.12, find sin A, cos A, tan A, sin B, cos B and tan B.

Triangle ABC is shown in Fig. 19.14. √ By Pythagoras’ theorem, AC = 52 + 122 = 13 By definition:

B 5

3

and A

sin A =

opposite side 12 = hypotenuse 13

or

0.9231

cos C =

adjacent side 12 = hypotenuse 13

or

0.9231

tan A =

opposite side 12 = adjacent side 5

or

2.400

C

Fig. 19.12

3. If cos A = 4. If tan X =

A 5 cm

15 find sin A and tan A, in fraction form. 17 15 , find sin X and cos X , in fraction form. 112

5. For the right-angled triangle shown in Fig. 19.13, find: (a) sin α (b) cos θ (c) tan θ

a 8

B

12 cm

C

Fig. 19.14

Problem 9. In triangle PQR shown in Fig. 19.15, find the lengths of PQ and PR. P

17 u 15

Fig. 19.13 Q

6. If tan θ =

7 , find sin θ and cos θ in fraction form. 24

Fig. 19.15

38° 7.5 cm

R

146

Basic Engineering Mathematics

tan 38◦ =

PQ PQ = QR 7.5

Z 20.0 mm

◦

PQ = 7.5 tan 38 = 7.5(0.7813) = 5.860 cm

hence

23°17′

X

QR 7.5 = cos 38 = PR PR 7.5 7.5 = 9.518 cm = hence PR = cos 38◦ 0.7880 [Check: Using Pythagoras’ theorem ◦

Y

Fig. 19.17

(Check: Using Pythagoras’ theorem

(7.5)2 + (5.860)2 = 90.59 = (9.518)2 ]

(18.37)2 + (7.906)2 = 400.0 = (20.0)2 ) Area of triangle XYZ = 12 (base)(perpendicular height)

Problem 10. Solve the triangle ABC shown in Fig. 19.16. 35 mm

A

=

1 (XY )(XZ) 2

= 12 (18.37)(7.906)

= 72.62 mm2

B

37 mm

Now try the following exercise

C

Fig. 19.16

Exercise 73

To ‘solve triangle ABC’ means ‘to find the length AC and angles B and C sin C =

35 = 0.94595 37

Further problems on the solution of rightangled triangles (Answers on page 278)

1. Solve triangle ABC in Fig. 19.18(i). 2. Solve triangle DEF in Fig. 19.18(ii).

hence C = sin−1 0.94595 = 71.08◦ or 71◦ 5 ◦

◦

◦

◦

B = 180 − 90 − 71 5 = 18 55 (since angles in a triangle add up to 180◦ )

AC = 37 sin 18◦ 55 = 37(0.3242) = 12.0 mm

or, using Pythagoras’ theorem, 372 = 352 + AC 2 , from which, AC = (372 − 352 ) = 12.0 mm.

A

35° 5.0 cm

F

∠Z = 180◦ − 90◦ − 23◦ 17 = 66◦ 43

XY cos 23 17 = 20.0 ◦

(iii)

Fig. 19.18

J

M

25°35′ P

K

3.69 m Q

N

8.75 m

32.0 mm 51° (i)

R

L (ii)

O

(iii)

Fig. 19.19

XZ = 20.0 sin 23◦ 17 = 20.0(0.3953) = 7.906 mm

hence

I (ii)

4. Solve the triangle JKL in Fig. 19.19(i) and find its area.

It is always advisable to make a reasonably accurate sketch so as to visualize the expected magnitudes of unknown sides and angles. Such a sketch is shown in Fig. 19.17.

hence

H 41° 15.0 mm

C

6.7 mm

XZ 20.0

G

3. Solve triangle GHI in Fig. 19.18(iii).

Problem 11. Solve triangle XYZ given ∠X = 90◦ , ∠Y = 23◦ 17 and YZ = 20.0 mm. Determine also its area.

sin 23◦ 17 =

3 cm E

4 cm

(i)

AC sin B = 37 hence

D

B

◦

XY = 20.0 cos 23 17

= 20.0(0.9186) = 18.37 mm

5. Solve the triangle MNO in Fig. 19.19(ii) and find its area. 6. Solve the triangle PQR in Fig. 19.19(iii) and find its area. 7. A ladder rests against the top of the perpendicular wall of a building and makes an angle of 73◦ with the ground.

Introduction to trigonometry

If the foot of the ladder is 2 m from the wall, calculate the height of the building. 8. Determine the length x in Figure 19.20.

147

Figure 19.23 shows the pylon AB and the angle of elevation of A from point C is 23◦ tan 23◦ =

AB AB = BC 80

Hence height of pylon AB = 80 tan 23◦ = 80(0.4245) 10 mm

56°

= 33.96 m = 34 m to the nearest metre A

x

Fig. 19.20 23°

C

B

80 m

19.5 Angles of elevation and depression Fig. 19.23 (a) If, in Fig. 19.21, BC represents horizontal ground and AB a vertical flagpole, then the angle of elevation of the top of the flagpole, A, from the point C is the angle that the imaginary straight line AC must be raised (or elevated) from the horizontal CB, i.e. angle θ.

Problem 13. A surveyor measures the angle of elevation of the top of a perpendicular building as 19◦ . He moves 120 m nearer the building and finds the angle of elevation is now 47◦ . Determine the height of the building.

A

C

u

The building PQ and the angles of elevation are shown in Fig. 19.24. B

In triangle PQS, tan 19◦ =

Fig. 19.21

h x + 120

hence h = tan 19◦ (x + 120), i.e. h = 0.3443(x + 120) P

f

(1)

P h

Q

R

R

47° Q

Fig. 19.22

x

19°

S

120

Fig. 19.24 (b) If, in Fig. 19.22, PQ represents a vertical cliff and R a ship at sea, then the angle of depression of the ship from point P is the angle through which the imaginary straight line PR must be lowered (or depressed) from the horizontal to the ship, i.e. angle φ. (Note, ∠PRQ is also φ – alternate angles between parallel lines.)

In triangle PQR, tan 47◦ = hence h = tan 47◦ (x),

i.e.

h x h = 1.0724 x

Equating equations (1) and (2) gives: 0.3443(x + 120) = 1.0724 x 0.3443x + (0.3443)(120) = 1.0724 x

Problem 12. An electricity pylon stands on horizontal ground. At a point 80 m from the base of the pylon, the angle of elevation of the top of the pylon is 23◦ . Calculate the height of the pylon to the nearest metre.

(0.3443)(120) = (1.0724 − 0.3443)x 41.316 = 0.7281x x=

41.316 = 56.74 m 0.7281

(2)

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Basic Engineering Mathematics

From equation (2), height of building,

2. If the angle of elevation of the top of a vertical 30 m high aerial is 32◦ , how far is it to the aerial?

h = 1.0724 x = 1.0724(56.74) = 60.85 m Problem 14. The angle of depression of a ship viewed at a particular instant from the top of a 75 m vertical cliff is 30◦ . Find the distance of the ship from the base of the cliff at this instant. The ship is sailing away from the cliff at constant speed and 1 minute later its angle of depression from the top of the cliff is 20◦ . Determine the speed of the ship in km/h.

Figure 19.25 shows the cliff AB, the initial position of the ship at C and the final position at D. Since the angle of depression is initially 30◦ then ∠ACB = 30◦ (alternate angles between parallel lines). tan 30◦ =

75 AB = BC BC

BC =

hence

75 75 = tan 30◦ 0.5774

= 129.9 m = initial position of ship from base of cliff

A

30° 20° 75 m 30°

B

C

20° x

D

Fig. 19.25

In triangle ABD,

75 75 AB = = tan 20 = BD BC + CD 129.9 + x ◦

129.9 + x =

Hence from which

75 75 = 206.0 m = tan 20◦ 0.3640

3. From the top of a vertical cliff 90.0 m high the angle of depression of a boat is 19◦ 50 . Determine the distance of the boat from the cliff. 4. From the top of a vertical cliff 80.0 m high the angles of depression of two buoys lying due west of the cliff are 23◦ and 15◦ , respectively. How far are the buoys apart? 5. From a point on horizontal ground a surveyor measures the angle of elevation of the top of a flagpole as 18◦ 40 . He moves 50 m nearer to the flagpole and measures the angle of elevation as 26◦ 22 . Determine the height of the flagpole. 6. A flagpole stands on the edge of the top of a building. At a point 200 m from the building the angles of elevation of the top and bottom of the pole are 32◦ and 30◦ respectively. Calculate the height of the flagpole. 7. From a ship at sea, the angles of elevation of the top and bottom of a vertical lighthouse standing on the edge of a vertical cliff are 31◦ and 26◦ , respectively. If the lighthouse is 25.0 m high, calculate the height of the cliff. 8. From a window 4.2 m above horizontal ground the angle of depression of the foot of a building across the road is 24◦ and the angle of elevation of the top of the building is 34◦ . Determine, correct to the nearest centimetre, the width of the road and the height of the building. 9. The elevation of a tower from two points, one due east of the tower and the other due west of it are 20◦ and 24◦ , respectively, and the two points of observation are 300 m apart. Find the height of the tower to the nearest metre.

x = 206.0 − 129.9 = 76.1 m

Thus the ship sails 76.1 m in 1 minute, i.e. 60 s, hence speed of ship 76.1 distance = m/s time 60 76.1 × 60 × 60 km/h = 4.57 km/h = 60 × 1000

=

Now try the following exercise Exercise 74 Further problems on angles of elevation and depression (Answers on page 278) 1. A vertical tower stands on level ground. At a point 105 m from the foot of the tower the angle of elevation of the top is 19◦ . Find the height of the tower.

19.6 Evaluating trigonometric ratios of any angles Four-figure tables are available which gives sines, cosines, and tangents, for angles between 0◦ and 90◦ . However, the easiest method of evaluating trigonometric functions of any angle is by using a calculator. The following values, correct to 4 decimal places, may be checked: sine 18◦ = 0.3090 cosine 56◦ = 0.5592 ◦ tangent 29 = 0.5543 sine 172◦ = 0.1392 cosine 115◦ = −0.4226 ◦ tangent 178 = −0.0349 sine 241.63◦ = −0.8799 cosine 331.78◦ = 0.8811 ◦ tangent 296.42 = −2.0127

Introduction to trigonometry

To evaluate, say, sine 42◦ 23 using a calculator means finding 23◦ since there are 60 minutes in 1 degree. sine 42 60 23 ˙ = 0.3833, 60

149

Problem 19. Determine the acute angles: (a) sin −1 0.7321 (b) cos −1 0.4174 (c) tan −1 1.4695

thus 42◦ 23 = 42.3833˙ ◦

Thus sine 42◦ 23 = sine 42.3833◦ = 0.6741, correct to 4 decimal places. Similarly, cosine 72◦ 38 = cosine 72 decimal places.

38◦ = 0.2985, correct to 4 60

(a) sin−1 θ is an abbreviation for ‘the angle whose sine is equal to θ’. 0.7321 is entered into a calculator and then the inverse sine (or sin−1 ) key is pressed. Hence sin−1 0.7321 = 47.06273 . . .◦ . Subtracting 47 leaves 0.06273 . . . ◦ and multiplying this by 60 gives 4 to the nearest minute. Hence sin−1 0.7321 = 47.06◦ or 47◦ 4

Problem 15. Evaluate correct to 4 decimal places: (a) sine 11◦ (b) sine 121.68◦ (c) sine 259◦ 10

Alternatively, in radians, sin−1 0.7321 = 0.821 radians. (b) cos−1 0.4174 = 65.33◦ or 65◦ 20 or 1.140 radians.

(a) sine 11◦ = 0.1908

(c) tan −1 1.4695 = 55.76◦ or 55◦ 46 or 0.973 radians.

(b) sine 121.68◦ = 0.8510 (c) sine 259◦ 10 = sine 259

10◦ = −0.9822 60

Problem 16. Evaluate, correct to 4 decimal places: (a) cosine 23◦ (b) cosine 159.32◦ (c) cosine 321◦ 41

Problem 20. Evaluate the following expression, correct to 4 significant figures: 4.2 tan 49◦ 26 − 3.7 sin 66◦ 1 7.1 cos 29◦ 34

By calculator:

(a) cosine 23◦ = 0.9205 (b) cosine 159.32◦ = −0.9356 (c) cosine 321◦ 41 = cosine 321

41◦ = 0.7846 60

Problem 17. Evaluate, correct to 4 significant figures: (a) tangent 276◦ (b) tangent 131.29◦ (c) tangent 76◦ 58

26 ◦ = 1.1681, tan 49◦ 26 = tan 49 60 sin 66◦ 1 = 0.9137

Hence =

(c) tangent 76◦ 58 = tan 76

Problem 18. (a) sin 1.481

58◦ = 4.320 60

Evaluate, correct to 4 significant figures: (b) cos(3π/5) (c) tan 2.93

cos 29◦ 34 = 0.8698

4.2 tan 49◦ 26 − 3.7 sin 66◦ 1 7.1 cos 29◦ 34

=

(4.2 × 1.1681) − (3.7 × 0.9137) (7.1 × 0.8698)

=

4.9060 − 3.3807 1.5253 = 6.1756 6.1756

(a) tangent 276◦ = −9.514 (b) tangent 131.29◦ = −1.139

and

= 0.2470 = 0.247,

Problem 21.

correct to 3 significant figures.

Evaluate correct to 4 decimal places:

(a) sin(−112◦ ) (b) cosine (−93◦ 16 ) (c) tangent (−217.29◦ )

(a) sin 1.481 means the sine of 1.481 radians. Hence a calculator needs to be on the radian function. Hence sin 1.481 = 0.9960 (b) cos(3π/5) = cos 1.884955 . . . = −0.3090 (c) tan 2.93 = −0.2148

(a) Positive angles are considered by convention to be anticlockwise and negative angles as clockwise. From Fig. 19.26, −112◦ is actually the same as +248◦ (i.e. 360◦ − 112◦ ). Hence, by calculator, sin(−112◦ ) = sin 248◦ = −0.9272

150

Basic Engineering Mathematics

90°

4. (a) sine

180° 112°

0° 360°

2π 3

(b) cos 1.681

(c) tan 3.672

In Problems 5 to 7, determine the acute angle in degrees (correct to 2 decimal places), degrees and minutes, and in radians (correct to 3 decimal places). 5. sin−1 0.2341

6. cos−1 0.8271

7. tan−1 0.8106

In Problems 8 to 10, evaluate correct to 4 significant figures. 270°

8. 4 cos 56◦ 19 − 3 sin 21◦ 57

Fig. 19.26 16 ◦ (b) cosine (−93◦ 16 ) = cosine −93 = −0.0570 60

9.

11.5 tan 49◦ 11 − sin 90◦ 3 cos 45◦

(c) tangent (−217.29◦ ) = −0.7615 (which is the same as tan(360◦ − 217.29◦ ), i.e. tan 141.71◦ )

10.

5 sin 86◦ 3 3 tan 14◦ 29 − 2 cos 31◦ 9

Now try the following exercise

11. Determine the acute angle, in degrees and minutes, correct to the nearest minute, given by ◦

−1 4.32 sin 42 16 sin 7.86

Exercise 75 Further problems on evaluating trigonometric ratios of any angle (Answers on page 278)

( sin 34◦ 27 )( cos 69◦ 2 ) (2 tan 53◦ 39 ) correct to 4 significant figures

12. Evaluate In Problems 1 to 4, evaluate correct to 4 decimal places: 1. (a) sine 27◦

(b) sine 172.41◦

(c) sine 302◦ 52

2. (a) cosine 124◦ (b) cosine 21.46◦ (c) cosine 284◦ 10 3. (a) tangent 145◦ (b) tangent 310.59◦ (c) tangent 49◦ 16

13. Evaluate correct to 4 decimal places: (a) sine(−125◦ ) (c) cos(−49◦ 15 )

(b) tan(−241◦ )

20 Trigonometric waveforms

20.1 Graphs of trigonometric functions (a) 1.0

By drawing up tables of values from 0◦ to 360◦ , graphs of y = sin A, y = cos A and y = tan A may be plotted. Values obtained with a calculator (correct to 3 decimal places – which is more than sufficient for plotting graphs), using 30◦ intervals, are shown below, with the respective graphs shown in Fig. 20.1.

0.5

0

0 0

30 60

90 120 150

210 240 270 300 330

360

A°

180

⫺0.5

(a) y = sin A A sin A

y ⫽ sin A

y

⫺1.0

30◦ 0.500

60◦ 0.866

90◦ 1.000

120◦ 0.866

150◦ 0.500

180◦ 0

(b) 1.0 y

y ⫽ cos A

0.5

A 210◦ 240◦ 270◦ 300◦ 330◦ 360◦ sin A −0.500 −0.866 −1.000 −0.866 −0.500 0

0

(b) y = cosA A cos A

30 60 90 120 150 180 210 240 270 300 330 360

A°

⫺0.5 ⫺1.0

0 30◦ 60◦ 90◦ 120◦ 150◦ 180◦ 1.000 0.866 0.500 0 −0.500 −0.866 −1.000

(c) 4

y ⫽ tan A

y 2

A cos A

210◦ −0.866

240◦ −0.500

270◦ 0

300◦ 0.500

330◦ 0.866

0 0

0

30 60 90 120

330 180 210 240 270 300

360

A°

⫺2

(c) y = tanA A tan A

150

360◦ 1.000

⫺4

30◦ 0.577

60◦ 1.732

90◦ ∞

120◦ −1.732

150◦ −0.577

180◦ 0

Fig. 20.1 From Fig. 20.1 it is seen that:

A tan A

210◦ 0.577

240◦ 1.732

270◦ ∞

300◦ −1.732

330◦ −0.577

360◦ 0

(i) Sine and cosine graphs oscillate between peak values of ± 1 (ii) The cosine curve is the same shape as the sine curve but displaced by 90◦ .

152

Basic Engineering Mathematics

(iii) The sine and cosine curves are continuous and they repeat at intervals of 360◦ ; the tangent curve appears to be discontinuous and repeats at intervals of 180◦ .

20.2 Angles of any magnitude Figure 20.2 shows rectangular axes XX and YY intersecting at origin 0. As with graphical work, measurements made to the right and above 0 are positive, while those to the left and downwards are negative. Let 0A be free to rotate about 0. By convention, when 0A moves anticlockwise angular measurement is considered positive, and vice versa. 90° Y Quadrant 2 ⫹

180°

0

A

⫺

⫺

Quadrant 3

sin θ2 =

+ =+ +

tan θ2 =

+ =− −

cos θ2 =

− =− +

Let 0A be further rotated so that θ3 is any angle in the third quadrant and let AD be constructed to form the right-angled triangle 0AD. Then sin θ3 =

− =− +

tan θ3 =

− =+ −

cos θ3 =

− =− +

Let 0A be further rotated so that θ4 is any angle in the fourth quadrant and let AE be constructed to form the right-angled triangle 0AE. Then

Quadrant 1 ⫹

⫺ X′

triangle 0AC. Then

⫹ X

0° 360°

sin θ4 =

− =− +

tan θ4 =

− =− +

cos θ4 =

+ =+ +

The above results are summarized in Fig. 20.4. The letters underlined spell the word CAST when starting in the fourth quadrant and moving in an anticlockwise direction.

Quadrant 4 Y′ 270°

90°

Fig. 20.2 Sine

Let 0A be rotated anticlockwise so that θ1 is any angle in the first quadrant and let perpendicular AB be constructed to form the right-angled triangle 0AB in Fig. 20.3. Since all three sides of the triangle are positive, the trigonometric ratios sine, cosine and tangent will all be positive in the first quadrant. (Note: 0A is always positive since it is the radius of a circle). Let 0A be further rotated so that θ2 is any angle in the second quadrant and let AC be constructed to form the right-angled

⫹

Tangent

180°

⫺

270°

A ⫹

0

⫹

u4 ⫹

E B ⫺

A Quadrant 4 270°

Fig. 20.4

⫹

u1 ⫹

A Quadrant 3

Fig. 20.3

Cosine

Quadrant 1

⫹

D ⫺ u2 u3 C

0° 360°

180°

90° Quadrant 2 A

All positive

0° 360°

In the first quadrant of Fig. 20.1 all of the curves have positive values; in the second only sine is positive; in the third only tangent is positive; in the fourth only cosine is positive – exactly as summarized in Fig. 20.4. A knowledge of angles of any magnitude is needed when finding, for example, all the angles between 0◦ and 360◦ whose sine is, say, 0.3261. If 0.3261 is entered into a calculator and then the inverse sine key pressed (or sin-key) the answer 19.03◦ appears. However, there is a second angle

Trigonometric waveforms

between 0◦ and 360◦ which the calculator does not give. Sine is also positive in the second quadrant [either from CAST or from Fig. 20.1(a)]. The other angle is shown in Fig. 20.5 as angle θ where θ = 180◦ − 19.03◦ = 160.97◦ . Thus 19.03◦ and 160.97◦ are the angles between 0◦ and 360◦ whose sine is 0.3261 (check that sin 160.97◦ = 0.3261 on your calculator).

153

90° S

180°

A

u

0° 360°

u

T

C 270°

90°

Fig. 20.7 S

A u

180°

19.03°

19.03°

T

0° 360°

Problem 2. Determine all the angles between 0◦ and 360◦ whose tangent is 1.7629

C

A tangent is positive in the first and third quadrants – see Fig. 20.8. From Fig. 20.9, θ = tan−1 1.7629 = 60.44◦

270°

Fig. 20.5

y ⫽ tan x y

Be careful! Your calculator only gives you one of these answers. The second answer needs to be deduced from a knowledge of angles of any magnitude, as shown in the following worked problems.

1.7629 0

180° 270° 240.44°

90° 60.44°

360° x

Problem 1. Determine all the angles between 0◦ and 360◦ whose sine is −0.4638 The angles whose sine is −0.4638 occurs in the third and fourth quadrants since sine is negative in these quadrants – see Fig. 20.6.

Fig. 20.8

90° y

y ⫽ sin x

S

1.0

A u

207.63° 0 ⫺0.4638

90° 180°

270°

u

360° x

⫺1.0

0 360°

180°

332.37°

T

C 270°

Fig. 20.9

Fig. 20.6

From Fig. 20.7, θ = sin−1 0.4638 = 27.63◦ . Measured from 0◦ , the two angles between 0◦ and 360◦ whose sine is −0.4638 are 180◦ + 27.63◦ , i.e. 207.63◦ and 360◦ − 27.63◦ , i.e. 332.37◦ (Note that a calculator only gives one answer, i.e. −27.632588◦ )

Measured from 0◦ , the two angles between 0◦ and 360◦ whose tangent is 1.7629 are 60.44◦ and 180◦ + 60.44◦ , i.e. 240.44◦ Problem 3. Solve the equation cos−1 (−0.2348) = α for angles of α between 0◦ and 360◦ .

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Basic Engineering Mathematics

Cosine is positive in the first and fourth quadrants and thus negative in the second and third quadrants – from Fig. 20.5 or from Fig. 20.1(b). In Fig. 20.10, angle θ = cos−1 (0.2348) = 76.42◦

2. Solve the following equations for values of x between 0◦ and 360◦ : (a) x = cos−1 0.8739

(b) x = cos−1 (−0.5572)

3. Find the angles between 0◦ to 360◦ whose tangent is: 90°

(a) 0.9728 A

S u

180°

20.3 The production of a sine and cosine wave

0° 360°

u T

In Fig. 20.11, let OR be a vector 1 unit long and free to rotate anticlockwise about O. In one revolution a circle is produced and is shown with 15◦ sectors. Each radius arm has a vertical and a horizontal component. For example, at 30◦ , the vertical component is TS and the horizontal component is OS. From trigonometric ratios,

C

270°

Fig. 20.10

Measured from 0◦ , the two angles whose cosine is −0.2348 are α = 180◦ − 76.42◦ i.e. 103.58◦ and α = 180◦ + 76.42◦ , i.e. 256.42◦

Now try the following exercise Exercise 76 Further problems on angles of any magnitude (Answers on page 279) 1. Determine all of the angles between 0◦ and 360◦ whose sine is: (a) 0.6792

(b) −2.3418

(b) −0.1483

sin 30◦ =

TS TS = , TO 1

i.e. TS = sin 30◦

and cos 30◦ =

OS OS = , TO 1

i.e. OS = cos 30◦

The vertical component TS may be projected across to T S , which is the corresponding value of 30◦ on the graph of y against angle x◦ . If all such vertical components as TS are projected on to the graph, then a sine wave is produced as shown in Fig. 20.11. If all horizontal components such as OS are projected on to a graph of y against angle x◦ , then a cosine wave is produced. It is easier to visualize these projections by redrawing the circle with the radius arm OR initially in a vertical position as shown in Fig. 20.12. From Figs. 20.11 and 20.12 it is seen that a cosine curve is of the same form as the sine curve but is displaced by 90◦ (or π/2 radians).

y 90° 120°

y ⫽ sin x T

150°

0.5 R

180°

0

S

360°

330°

210° 240° 270°

Fig. 20.11

1.0

60°

300° ⫺1.0

T′ Angle x °

S′ 30° 60° ⫺0.5

120°

210°

270°

330°

Trigonometric waveforms

155

y 15° 0°R T

330° 315°

S

45°

1.0

S′

y ⫽ cos x

0.5

60°

Angle x °

285° 90°

0° 255°

0

O′ 30° 60°

120°

240°

180°

300°

360°

⫺0.5

120°

225° 150°

⫺1.0

210°

180°

Fig. 20.12 (ii) A graph of y = sin 12 A is shown in Fig. 20.14 using the following table of values.

20.4 Sine and cosine curves Graphs of sine and cosine waveforms (i) A graph of y = sin A is shown by the broken line in Fig. 20.13 and is obtained by drawing up a table of values as in Section 20.1. A similar table may be produced for y = sin 2A. A◦ 2A sin 2A ◦

0 0 0

30 60 0.866

A 2A sin 2A

135 270 −1.0

A◦ 2A sin 2A

270 540 0

45 90 1.0

150 300 −0.866 300 600 −0.866

60 120 0.866 180 360 0

90 180 0

0

360 720 0

0

⫺1.0

Fig. 20.13

180°

240 120 0.866

270 135 0.707

y ⫽ sin A

y

330 660 −0.866

90°

180°

300 150 0.500

330 165 0.259

360 180 0

1

y ⫽ sin 2 A

270°

360°

A°

⫺1.0

Fig. 20.14 y ⫽ sin 2A

90°

210 105 0.966

240 480 0.866

y ⫽ sin A

1.0

A◦ 1 A 2 sin 12 A

225 450 1.0

A graph of y = sin 2A is shown in Fig. 20.13. y

0 30 60 90 120 150 180 0 15 30 45 60 75 90 0 0.259 0.500 0.707 0.866 0.966 1.00

1.0

210 420 0.866

315 630 −1.0

120 240 −0.866

A◦ 1 A 2 sin 12 A

270°

360°

(iii) A graph of y = cos A is shown by the broken line in Fig. 20.15 and is obtained by drawing up a table of values. A similar table may be produced for y = cos 2A with the result as shown. A°

(iv) A graph of y = cos 12 A is shown in Fig. 20.16 which may be produced by drawing up a table of values, similar to above.

Periodic time and period (i) Each of the graphs shown in Figs. 20.13 to 20.16 will repeat themselves as angle A increases and are thus called periodic functions.

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Basic Engineering Mathematics

y 1.0

Amplitude = 1 and period = 360◦ /3 = 120◦ .

y ⫽ cos 2A

y ⫽ cos A

A sketch of y = sin 3A is shown in Fig. 20.17.

y 0

90°

180°

270°

360°

1.0

A°

0

⫺1.0

y y ⫽ cos

1 2

A

180°

270°

360°

A°

Fig. 20.17

y ⫽ cos A

Problem 5. radians 0

90°

⫺1.0

Fig. 20.15

1.0

y ⫽ sin 3A

90°

180°

270°

360°

Sketch y = 3 sin 2A from A = 0 to A = 2π

A°

Amplitude = 3 and period = 2π/2 = π rads (or 180◦ ) A sketch of y = 3 sin 2A is shown in Fig. 20.18. ⫺1.0 y

Fig. 20.16 (ii) y = sin A and y = cos A repeat themselves every 360◦ (or 2π radians); thus 360◦ is called the period of these waveforms. y = sin 2A and y = cos 2A repeat themselves every 180◦ (or π radians); thus 180◦ is the period of these waveforms. (iii) In general, if y = sin pA or y = cos pA (where p is a constant) then the period of the waveform is 360◦ /p (or 2π/p rad). Hence if y = sin 3A then the period is 360/3, i.e. 120◦ , and if y = cos 4A then the period is 360/4, i.e. 90◦

3

0

y ⫽ 3 sin 2A

90°

270°

180°

360°

⫺3

Fig. 20.18

Problem 6.

Sketch y = 4 cos 2x from x = 0◦ to x = 360◦

Amplitude Amplitude is the name given to the maximum or peak value of a sine wave. Each of the graphs shown in Figs. 20.13 to 20.16 has an amplitude of +1 (i.e. they oscillate between +1 and −1). However, if y = 4 sin A, each of the values in the table is multiplied by 4 and the maximum value, and thus amplitude, is 4. Similarly, if y = 5 cos 2A, the amplitude is 5 and the period is 360◦ /2, i.e. 180◦ Problem 4. Sketch y = sin 3A between A = 0◦ and A = 360◦

A°

Amplitude = 4 and period = 360◦ /2 = 180◦ . A sketch of y = 4 cos 2x is shown in Fig. 20.19. Problem 7.

Sketch y = 2 sin 35 A over one cycle.

Amplitude = 2; period =

360◦ 3 5

=

360◦ × 5 = 600◦ . 3

A sketch of y = 2 sin 35 A is shown in Fig. 20.20.

Trigonometric waveforms

(iii) By drawing up a table of values, a graph of y = cos(A + 45◦ ) may be plotted as shown in Fig. 20.22. If y = cos A is assumed to start at 0◦ then y = cos(A + 45◦ ) starts 45◦ earlier (i.e. has a maximum value 45◦ earlier). Thus y = cos(A + 45◦ ) is said to lead y = cos A by 45◦

y y ⫽ 4 cos 2x

4

0

90°

157

180°

360°

270°

x° 45°

⫺4

y ⫽ cos A y ⫽ cos (A ⫹ 45°) y

Fig. 20.19 0

y 2

y ⫽ 2 sin

3 5

90°

180°

270°

360°

A°

A ⫺1.0 45°

0

180°

600°

540°

360°

A°

Fig. 20.22 (iv) Generally, a graph of y = sin(A − α) lags y = sin A by angle α, and a graph of y = sin(A + α) leads y = sin A by angle α

⫺2

Fig. 20.20

(v) A cosine curve is the same shape as a sine curve but starts 90◦ earlier, i.e. leads by 90◦ . Hence cos A = sin(A + 90◦ )

Lagging and leading angles (i) A sine or cosine curve may not always start at 0◦ . To show this a periodic function is represented by y = sin(A ± α) or y = cos(A ± α) where α is a phase displacement compared with y = sin A or y = cos A. (ii) By drawing up a table of values, a graph of y = sin(A − 60◦ ) may be plotted as shown in Fig. 20.21. If y = sin A is assumed to start at 0◦ then y = sin(A − 60◦ ) starts 60◦ later (i.e. has a zero value 60◦ later). Thus y = sin(A − 60◦ ) is said to lag y = sin A by 60◦

Amplitude = 5 and period = 360◦ /1 = 360◦ . 5 sin(A + 30◦ ) leads 5 sin A by 30◦ (i.e. starts 30◦ earlier). A sketch of y = 5 sin(A + 30◦ ) is shown in Fig. 20.23. 30°

60°

y

Problem 8. Sketch y = 5 sin(A + 30◦ ) from A = 0◦ to A = 360◦

y ⫽ sin A

y

y ⫽ sin (A⫺60°)

1.0

5

y ⫽ 5 sin A y ⫽ 5 sin (A ⫹ 30°)

90°

0

180°

270°

360°

A°

0 30° ⫺5

⫺1.0 60°

Fig. 20.21

Fig. 20.23

90°

180°

270°

360°

A°

158

Basic Engineering Mathematics

Sketch y = 7 sin(2A − π/3) over one cycle.

Problem 9.

In general, y = sin( pt − α) lags y = sin pt by α/p, hence 7 sin(2A − π/3) lags 7 sin 2A by (π/3)/2, i.e. π/6 rad or 30◦ . A sketch of y = 7 sin(2A − π/3) is shown in Fig. 20.24.

7

0

⫺7

90° π/2

180° π

270° 3π/2

360° 2π

A°

π/6

Fig. 20.24 Problem 10. cycle.

3x 7 sin 2 8 7. y = 4 cos(2θ + 30◦ )

6. y = 6 sin(t − 45◦ )

5. y =

20.5 Sinusoidal form A sin(ωt ± α )

y ⫽ 7sin 2A y ⫽ 7sin (2A⫺π/3)

π/6

2. y = 2 sin

3. y = 3 sin 4t

Amplitude = 7 and period = 2π/2 = π radians.

y

5x 2 θ 4. y = 3 cos 2

1. y = cos 3A

In Fig. 20.26, let OR represent a vector that is free to rotate anticlockwise about O at a velocity of ω rad/s. A rotating vector is called a phasor. After a time t seconds OR will have turned through an angle ωt radians (shown as angle TOR in Fig. 20.26). If ST is constructed perpendicular to OR, then sin ωt = ST /OT , i.e. ST = OT sin ωt. If all such vertical components are projected on to a graph of y against ωt, a sine wave results of amplitude OR (as shown in Section 20.3). If phasor OR makes one revolution (i.e. 2π radians) in T seconds, then the angular velocity, ω = 2π/T rad/s, from which,

Sketch y = 2 cos(ωt − 3π/10) over one

T = 2π /ω seconds

T is known as the periodic time. The number of complete cycles occurring per second is called the frequency, f

Amplitude = 2 and period = 2π/ω rad.

Frequency =

2 cos(ωt − 3π/10) lags 2 cos ωt by 3π/10ω seconds. A sketch of y = 2 cos(ωt − 3π/10) is shown in Fig. 20.25. 3π/10v rads

i.e.

1 ω number of cycles = = Hz second T 2π

f =

ω Hz 2π

y 2

Hence angular velocity,

y ⫽ 2cos vt y ⫽ 2cos(vt⫺3π/10)

0

π/2v

π/v

3π/2v

2π/v

t

⫺2

Fig. 20.25

Now try the following exercise Exercise 77 Further problems on sine and cosine curves (Answers on page 279) In Problems 1 to 7 state the amplitude and period of the waveform and sketch the curve between 0◦ and 360◦ .

ω = 2πf rad/s

Amplitude is the name given to the maximum or peak value of a sine wave, as explained in Section 20.4. The amplitude of the sine wave shown in Fig. 20.26 has an amplitude of 1. A sine or cosine wave may not always start at 0◦ . To show this a periodic function is represented by y = sin(ωt ± α) or y = cos(ωt ± α), where α is a phase displacement compared with y = sin A or y = cos A. A graph of y = sin(ωt − α) lags y = sin ωt by angle α, and a graph of y = sin(ωt + α) leads y = sin ωt by angle α. The angle ωt is measured in radians rad i.e. ω (t s) = ωt radians s hence angle α should also be in radians. The relationship between degrees and radians is: 360◦ = 2π radians or

180◦ = π radians

Trigonometric waveforms

159

y v rads/s

y ⫽ sin vt

1.0

T vt 0

S R

0

vt

90°

180°

270°

π/2

π

3π/2

360° 2π vt

⫺1.0

Fig. 20.26 Hence 1 rad =

180 = 57.30◦ and, for example, π

π = 1.239 rad 71 = 71 × 180 ◦

Amplitude = maximum displacement = 2.5 m Angular velocity, ω = 2πf = 2π(60) = 120π rad/s Hence displacement = 2.5 sin(120πt + α) m

Given a general sinusoidal function y = A sin(ωt ± α), then (i) A = amplitude

When t = 0, displacement = 90 cm = 0.90 m Hence 0.90 = 2.5 sin(0 + α)

(ii) ω = angular velocity = 2πf rad/s 2π = periodic time T seconds (iii) ω ω (iv) = frequency, f hertz 2π (v) α = angle of lead or lag (compared with y = A sin ωt)

Problem 11. An alternating current is given by i = 30 sin (100πt + 0.27) amperes. Find the amplitude, periodic time, frequency and phase angle (in degrees and minutes).

i.e.

sin α =

0.90 = 0.36 2.5

Hence α = sin−1 0.36 = 21.10◦ = 21◦ 6 = 0.368 rad Thus displacement = 2.5 sin(120π t + 0.368) m Problem 13. The instantaneous value of voltage in an a.c. circuit at any time t seconds is given by v = 340 sin (50πt − 0.541) volts. Determine: (a) the amplitude, periodic time, frequency and phase angle (in degrees) (b) the value of the voltage when t = 0 (c) the value of the voltage when t = 10 ms

i = 30 sin (100πt + 0.27)A, hence amplitude = 30 A

(d) the time when the voltage first reaches 200 V, and

Angular velocity ω = 100π , hence

(e) the time when the voltage is a maximum

2π 1 2π = = = 0.02 s or 20 ms ω 100π 50 1 1 = 50 Hz Frequency, f = = T 0.02 180 ◦ Phase angle, α = 0.27 rad = 0.27 × π

periodic time, T =

= 15.47

◦

or

◦

Sketch one cycle of the waveform. (a) Amplitude = 340V Angular velocity, ω = 50π Hence periodic time, T =

15 28 leading

2π 2π 1 = = ω 50π 25

= 0.04 s

i = 30 sin(100π t)

or

40 ms

1 1 = = 25 Hz T 0.04 180 Phase angle = 0.541 rad = 0.541 × π

Frequency f = Problem 12. An oscillating mechanism has a maximum displacement of 2.5 m and a frequency of 60 Hz. At time t = 0 the displacement is 90 cm. Express the displacement in the general form A sin (ωt ± α).

= 31◦ lagging v = 340 sin(50πt)

160

Basic Engineering Mathematics

(b) When t = 0, v = 340 sin(0 − 0.541)

Now try the following exercise

◦

= 340 sin(−31 ) = −175.1 V 10 (c) When t = 10 ms then v = 340 sin 50π 3 − 0.541 10 = 340 sin(1.0298) = 340 sin 59◦ = 291.4 volts (d) When v = 200 volts then 200 = 340 sin(50π t − 0.541) 200 = sin(50πt − 0.541) 340 Hence (50π t − 0.541) = sin−1

200 340

0.6288 rad

= 1.1698

1.1698 = 7.447 ms 50π (e) When the voltage is a maximum, v = 340 V time, t =

or

1.5708 rad

50πt = 1.5708 + 0.541 = 2.1118 2.1118 = 13.44 m 50π

A sketch of v = 340 sin(50πt − 0.541) volts is shown in Fig. 20.27 Voltage v

⫺175.1 ⫺340

Fig. 20.27

v ⫽ 340 sin(50πt⫺0.541) v ⫽ 340 sin 50πt

10

20

3. v = 300 sin(200πt − 0.412) V 4. A sinusoidal voltage has a maximum value of 120 V and a frequency of 50 Hz. At time t = 0, the voltage is (a) zero, and (b) 50 V.

i = 5 sin(100πt − 0.432) amperes

1 = sin(50π t − 0.541)

7.447 13.44

2. y = 75 sin(40t − 0.54) cm

7. The current in an a.c. circuit at any time t seconds is given by:

Hence 340 = 340 sin(50πt − 0.541)

0

1. i = 40 sin(50πt + 0.29) mA

6. An oscillating mechanism has a maximum displacement of 3.2 m and a frequency of 50 Hz. At time t = 0 the displacement is 150 cm. Express the displacement in the general form A sin(ωt ± α)

Hence when v = 200 V,

340 291.4 200

In Problems 1 to 3 find the amplitude, periodic time, frequency and phase angle (stating whether it is leading or lagging sin ωt) of the alternating quantities given.

5. An alternating current has a periodic time of 25 ms and a maximum value of 20 A. When time t = 0, current i = −10 amperes. Express the current i in the form i = A sin(ωt ± α)

50πt = 0.6288 + 0.541

Hence time, t =

Further problems on the sinusoidal form Asin(ωt ± α) (Answers on page 279)

Express the instantaneous voltage v in the form v = A sin(ωt ± α)

= 36.03◦ or

50πt − 0.541 = sin−1 1 = 90◦

Exercise 78

30

40 t (ms)

Determine (a) the amplitude, periodic time, frequency and phase angle (in degrees) (b) the value of current at t = 0 (c) the value of current at t = 8 ms (d) the time when the current is first a maximum (e) the time when the current first reaches 3A. Sketch one cycle of the waveform showing relevant points.

Trigonometric waveforms

Assignment 9 This assignment covers the material in Chapters 19 and 20. The marks for each question are shown in brackets at the end of each question. 1. FigureA9.1 shows a plan view of a kite design. Calculate the lengths of the dimensions shown as a and b (4) 2. In Fig. A9.1, evaluate (a) angle θ (b) angle α

(6)

3. Determine the area of the plan view of a kite shown in Fig. A9.1. (4)

161

4. If the angle of elevation of the top of a 25 m perpendicular building from point A is measured as 27◦ , determine the distance to the building. Calculate also the angle of elevation at a point B, 20 m closer to the building than point A. (5) 5. Evaluate, each correct to 4 significant figures: (a) sin 231.78◦

(b) cos 151◦ 16

(c) tan

3π 8

(3)

6. (a) Determine the acute angle cos−1 0.4117 (i) in degrees and minutes and (ii) in radians (correct to 2 decimal places). (b) Calculate the other angle (in degrees) between 0◦ (4) and 360◦ which satisfies cos−1 0.4117 7. Sketch the following curves labelling relevant points:

a

(a) y = 4 cos (θ + 45◦ )

20.0 cm

42.0 cm

a

(b) y = 5 sin (2t − 60◦ )

(8)

8. The current in an alternating current circuit at any time t seconds is given by: i = 120 sin (100πt + 0.274) amperes Determine

b

60.0 cm

(a) the amplitude, periodic time, frequency and phase angle (with reference to 120 sin 100πt) (b) the value of current when t = 0 (c) the value of current when t = 6 ms

u

Fig. A9.1

Sketch one cycle of the oscillation.

(16)

21 Cartesian and polar co-ordinates

21.1 Introduction from which There are two ways in which the position of a point in a plane can be represented. These are (a) by Cartesian co-ordinates, i.e. (x, y), and (b) by polar co-ordinates, i.e. (r, θ), where r is a ‘radius’ from a fixed point and θ is an angle from a fixed point.

21.2 Changing from Cartesian into polar co-ordinates In Fig. 21.1, if lengths x and y are known, then the length of r can be obtained from Pythagoras’ theorem (see Chapter 19) since OPQ is a right-angled triangle.

θ = tan−1

y x

y r = x2 + y2 and θ = tan−1 are the two formulae we need x to change from Cartesian to polar co-ordinates. The angle θ, which may be expressed in degrees or radians, must always be measured from the positive x-axis, i.e. measured from the line OQ in Fig. 21.1. It is suggested that when changing from Cartesian to polar co-ordinates a diagram should always be sketched. Problem 1. Change the Cartesian co-ordinates (3, 4) into polar co-ordinates. A diagram representing the point (3, 4) is shown in Fig. 21.2.

r 2 = (x2 + y2 )

Hence

from which,

r = x 2 + y2

P

y

r

4

y P

u O

r

3

y

Fig. 21.2

u O

x

Q

Fig. 21.1 From trigonometric ratios (see Chapter 19), y tan θ = x

x

x

√ From Pythagoras’ theorem, r = 32 + 42 = 5 (note that −5 has no meaning in this context). By trigonometric ratios, θ = tan−1 43 = 53.13◦ or 0.927 rad. [note that 53.13◦ = 53.13 × (π/180) rad = 0.927 rad.] Hence (3, 4) in Cartesian co-ordinates corresponds to (5, 53.13◦ ) or (5, 0.927 rad) in polar co-ordinates.

Cartesian and polar co-ordinates

Problem 2. tion (−4, 3)

y

Express in polar co-ordinates the posi5

u O

a

A diagram representing the point using the Cartesian co-ordinates (−4, 3) is shown in Fig. 21.3. 12

y

P

163

x

r

r

3

P u

a

O 4

Fig. 21.4 x y

Fig. 21.3 From Pythagoras’ theorem, r =

√ 42 + 32 = 5

By trigonometric ratios, α = tan−1

3 4

u

= 36.87◦ or 0.644 rad.

Hence

θ = 180◦ − 36.87◦ = 143.13◦

or

θ = π − 0.644 = 2.498 rad

2 O

5 r

Hence the position of point P in polar co-ordinate form is (5, 143.13◦ ) or (5, 2.498 rad) Express (−5, −12) in polar co-ordinates.

Problem 3.

A sketch showing the position (−5, −12) is shown in Fig. 21.4. r= and

12 5

= 67.38◦

◦

or

◦

Hence

θ = 180 + 67.38 = 247.38

or

θ = π + 1.176 = 4.318 rad

Fig. 21.5

Now try the following exercise

1.176 rad ◦

Thus (−5, −12) in Cartesian co-ordinates corresponds to (13, 247.38◦ ) or (13, 4.318 rad) in polar co-ordinates.

Problem 4.

P

Exercise 79

52 + 122 = 13

α = tan−1

x

a

Express (2, −5) in polar co-ordinates.

Further problems on changing from Cartesian into polar co-ordinates (Answers on page 279)

In Problems 1 to 8, express the given Cartesian co-ordinates as polar co-ordinates, correct to 2 decimal places, in both degrees and in radians. 1. (3, 5)

2. (6.18, 2.35)

3. (−2, 4)

4. (−5.4, 3.7)

5. (−7, −3)

6. (−2.4, −3.6)

7. (5, −3)

8. (9.6, −12.4)

A sketch showing the position (2, −5) is shown in Fig. 21.5. r= α=

22 + 5 2 =

tan−1 25 ◦

√ 29 = 5.385 correct to 3 decimal places

= 68.20

◦

or

1.190 rad

Hence θ = 360 − 68.20 = 291.80◦ or

◦

θ = 2π − 1.190 = 5.093 rad

Thus (2, −5) in Cartesian co-ordinates corresponds to (5.385, 291.80◦ ) or (5.385, 5.093 rad) in polar co-ordinates.

21.3 Changing from polar into Cartesian co-ordinates From the right-angled triangle OPQ in Fig. 21.6. x y cos θ = and sin θ = , from trigonometric ratios r r Hence

x = r cos θ

and

y = r sin θ

164

Basic Engineering Mathematics

y

which corresponds to length OA in Fig. 21.8. P

y = r sin θ = 6 sin 137◦ = 4.092

r

which corresponds to length AB in Fig. 21.8.

y u O

Q

x

x

(Note that when changing from polar to Cartesian co-ordinates it is not quite so essential to draw a sketch. Use of x = r cos θ and y = r sin θ automatically produces the correct signs.)

Fig. 21.6 If lengths r and angle θ are known then x = r cos θ and y = r sin θ are the two formulae we need to change from polar to Cartesian co-ordinates. Problem 5.

Thus (6, 137◦ ) in polar co-ordinates corresponds to (−4.388, 4.092) in Cartesian co-ordinates.

Change (4, 32◦ ) into Cartesian co-ordinates.

A sketch showing the position (4, 32◦ ) is shown in Fig. 21.7. Now

x = r cos θ = 4 cos 32◦ = 3.39

and

y = r sin θ = 4 sin 32◦ = 2.12

Problem 7. Express (4.5, 5.16 rad) in Cartesian co-ordinates A sketch showing the position (4.5, 5.16 rad) is shown in Fig. 21.9. x = r cos θ = 4.5 cos 5.16 = 1.948 which corresponds to length OA in Fig. 21.9. y

y

u ⫽ 5.16 rad A O

r⫽4

y

r ⫽ 4.5

u ⫽ 32⬚ O

x

x

x

B

Fig. 21.7 Fig. 21.9 ◦

Hence (4, 32 ) in polar co-ordinates corresponds to (3.39, 2.12) in Cartesian co-ordinates. Problem 6.

Express (6, 137◦ ) in Cartesian co-ordinates.

A sketch showing the position (6, 137◦ ) is shown in Fig. 21.8. x = r cos θ = 6 cos 137◦ = −4.388

r⫽6 u ⫽ 137⬚ A

Fig. 21.8

O

which corresponds to length AB in Fig. 21.9. Thus (1.948, −4.057) in Cartesian co-ordinates corresponds to (4.5, 5.16 rad) in polar co-ordinates.

21.4 Use of R → P and P → R functions on calculators

y

B

y = r sin θ = 4.5 sin 5.16 = −4.057

x

Another name for Cartesian co-ordinates is rectangular co-ordinates. Many scientific notation calculators possess R → P and P → R functions. The R is the first letter of the word rectangular and the P is the first letter of the word polar. Check the operation manual for your particular calculator to determine how to use these two functions. They make changing from Cartesian to polar co-ordinates, and vice-versa, so much quicker and easier.

Cartesian and polar co-ordinates

Now try the following exercise Exercise 80 Further problems on changing polar into Cartesian co-ordinates (Answers on page 279) In Problems 1 to 8, express the given polar co-ordinates as Cartesian co-ordinates, correct to 3 decimal places. 1. (5, 75◦ )

2. (4.4, 1.12 rad)

3. (7, 140◦ )

4. (3.6, 2.5 rad)

5. (10.8, 210◦ )

6. (4, 4 rad)

7. (1.5, 300◦ )

8. (6, 5.5 rad)

9. Figure 21.10 shows 5 equally spaced holes on an 80 mm pitch circle diameter. Calculate their co-ordinates relative to axes 0x and 0y in (a) polar form, (b) Cartesian form. y

O

Fig. 21.10

x

165

22 Areas of plane figures

22.1 Mensuration

A

B

D

C

Mensuration is a branch of mathematics concerned with the determination of lengths, areas and volumes.

Fig. 22.1

22.2 Properties of quadrilaterals Polygon

P

Q

S

R

A polygon is a closed plane figure bounded by straight lines. A polygon which has: (i) (ii) (iii) (iv) (v) (vi)

3 sides is called a triangle 4 sides is called a quadrilateral 5 sides is called a pentagon 6 sides is called a hexagon 7 sides is called a heptagon 8 sides is called an octagon

There are five types of quadrilateral, these being: (i) (ii) (iii) (iv) (v)

rectangle square parallelogram rhombus trapezium

(The properties of these are given below). If the opposite corners of any quadrilateral are joined by a straight line, two triangles are produced. Since the sum of the angles of a triangle is 180◦ , the sum of the angles of a quadrilateral is 360◦ . In a rectangle, shown in Fig. 22.1: (i) all four angles are right angles, (ii) opposite sides are parallel and equal in length, and (iii) diagonals AC and BD are equal in length and bisect one another.

Fig. 22.2

In a square, shown in Fig. 22.2: (i) (ii) (iii) (iv)

all four angles are right angles, opposite sides are parallel, all four sides are equal in length, and diagonals PR and QS are equal in length and bisect one another at right angles.

In a parallelogram, shown in Fig. 22.3: (i) opposite angles are equal, (ii) opposite sides are parallel and equal in length, and (iii) diagonals WY and XZ bisect one another. In a rhombus, shown in Fig. 22.4: (i) (ii) (iii) (iv) (v)

opposite angles are equal, opposite angles are bisected by a diagonal, opposite sides are parallel, all four sides are equal in length, and diagonals AC and BD bisect one another at right angles.

Areas of plane figures

W

X

167

(Continued)

Table 22.1 (vi) Circle

r Z

Y

Fig. 22.3

(vii) Semicircle A

b

d Area ⫽

r

B a

a

Area ⫽ πr 2 or

d

(viii) Sector of a circle

r

l Area ⫽

u r

b

D

2

u° (πr 2) or 12 r 2u 360° (u in rads)

Problem 1. State the types of quadrilateral shown in Fig. 22.6 and determine the angles marked a to l. F

E

d

E x

A

F

B

G

H

x J

40°

x

c

e

b

D

H

C

K

30°

x a

Fig. 22.5

G

(i)

f

M

L

(ii)

(iii)

In a trapezium, shown in Fig. 22.5:

115° S

(i) only one pair of sides is parallel

R N

22.3 Worked problems on areas of plane figures

l

O g

h 65° Q

35°

i U

52°

j

P

75°

(iv)

Table 22.1 x

k T

(v)

Fig. 22.6 x

Area ⫽ x 2

(ii) Rectangle

(i) ABCD is a square b

Area ⫽ l ⫻ b

l (iii) Parallelogram Area ⫽ b ⫻ h

h

(iv) Triangle h

Area ⫽

1 2

⫻b⫻h

In triangle FGH , 40◦ + 90◦ + b = 180◦ (angles in a triangle add up to 180◦ ) from which, b = 50◦. Also c = 40◦ (alternate angles between parallel lines EF and HG). (Alternatively, b and c are complementary, i.e. add up to 90◦ )

b a h b

The diagonals of a square bisect each of the right angles, hence 90◦ = 45◦ a= 2 (ii) EFGH is a rectangle

b

(v) Trapezium

πr 2 or πd 8

C

Fig. 22.4

(i) Square

1 2

πd 2 4

Area ⫽ 12 (a ⫹ b)h

d = 90◦ + c (external angle of a triangle equals the sum of the interior opposite angles), hence d = 90◦ + 40◦ = 130◦

Basic Engineering Mathematics

(iii) JKLM is a rhombus

50 mm A

hence e = 30◦

25 m

5 mm B

Thus ∠JKM = ∠MKL = ∠JMK = ∠LMK = 30◦ ,

6 mm 8 mm

2m

In triangle KLM , 30◦ + ∠KLM + 30◦ = 180◦ (angles in a triangle add up to 180◦ ), hence ∠KLM = 120◦ .

C 70 mm

The diagonal JL bisects ∠KLM , hence 120◦ = 60◦ f = 2

20 m

The diagonals of a rhombus bisect the interior angles and opposite internal angles are equal.

75 mm

168

(a)

(b)

Fig. 22.7

(iv) NOPQ is a parallelogram g = 52◦ (since opposite interior angles of a parallelogram are equal). In triangle NOQ, g + h + 65◦ = 180◦ (angles in a triangle add up to 180◦ ), from which, h = 180◦ − 65◦ − 52◦ = 63◦ ◦

i = 65 (alternate angles between parallel lines NQ and OP). j = 52◦ + i = 52◦ + 65◦ = 117◦ (external angle of a triangle equals the sum of the interior opposite angles). (v) RSTU is a trapezium 35◦ + k = 75◦ (external angle of a triangle equals the sum of the interior opposite angles), hence k = 40◦ ∠STR = 35◦ (alternate angles between parallel lines RU and ST ). l + 35◦ = 115◦ (external angle of a triangle equals the sum of the interior opposite angles), hence

(a) The girder may be divided into three separate rectangles as shown. Area of rectangle A = 50 × 5 = 250 mm2 Area of rectangle B = (75 − 8 − 5) × 6 = 62 × 6 = 372 mm2 Area of rectangle C = 70 × 8 = 560 mm2 Total area of girder = 250 + 372 + 560 = 1182 mm2 or 11.82 cm2 (b) Area of path = area of large rectangle − area of small rectangle = (25 × 20) − (21 × 16) = 500 − 336 = 164 m2

Problem 4. Find the area of the parallelogram shown in Fig. 22.8 (dimensions are in mm).

l = 115◦ − 35◦ = 80◦

B

A 15

Problem 2. A rectangular tray is 820 mm long and 400 mm wide. Find its area in (a) mm2 , (b) cm2 , (c) m2 . (a) Area = length × width = 820 × 400 = 328 000 mm2

D

h 25 34

C

E

Fig. 22.8

(b) 1 cm2 = 100 mm2 . Hence 328 000 mm2 =

328 000 cm2 = 3280 cm2 100

(c) 1 m2 = 10 000 cm2 . Hence 3280 cm2 =

3280 2 m = 0.3280 m2 10 000

Problem 3. Find (a) the cross-sectional area of the girder shown in Fig. 22.7(a) and (b) the area of the path shown in Fig. 22.7(b).

Area of parallelogram = base × perpendicular height. The perpendicular height h is found using Pythagoras’ theorem. BC 2 = CE 2 + h2 i.e. 152 = (34 − 25)2 + h2 h2 = 152 − 92 = 225 − 81 = 144 √ Hence, h = 144 = 12 mm (−12 can be neglected). Hence, area of ABCD = 25 × 12 = 300 mm2

Areas of plane figures

Problem 5. Figure 22.9 shows the gable end of a building. Determine the area of brickwork in the gable end. A 5m

5m

B C

D

169

(c) Circumference, c = 2πr, hence 70 35 c = = mm 2π 2π π 2 35 352 = Area of circle = πr 2 = π π π = 389.9 mm2 or 3.899 cm2 r=

6m

Problem 8. circles: 8m

Fig. 22.9

The shape is that of a rectangle and a triangle. Area of rectangle = 6 × 8 = 48 m2 CD = 4 m, AD = 5 m, hence AC = 3 m (since it is a 3, 4, 5 triangle). Hence, area of triangle ABD = 12 × 8 × 3 = 12 m2 Total area of brickwork = 48 + 12 = 60 m2 Problem 6. Determine the area of the shape shown in Fig. 22.10.

θ2 1 (πr 2 ) or r 2 θ (θ in radians). 360 2

(a) Area of sector 50 × π × 36 50 (π62 ) = = 5π = 15.71 cm2 360 360 (b) If diameter = 80 mm, then radius, r = 40 mm, and area of sector 42 107 107◦ 42 107.7 2 60 = (π40 ) = (π402 ) = (π 402 ) 360 360 360 = 1504 mm2 or 15.04 cm2 =

1 1 (c) Area of sector = r 2 θ = × 82 × 1.15 = 36.8 cm2 2 2

27.4 mm 5.5 mm 8.6 mm

Problem 9. A hollow shaft has an outside diameter of 5.45 cm and an inside diameter of 2.25 cm. Calculate the cross-sectional area of the shaft.

Fig. 22.10

The shape shown is a trapezium. 1 2

(a) having radius 6 cm with angle subtended at centre 50◦ (b) having diameter 80 mm with angle subtended at centre 107◦ 42 (c) having radius 8 cm with angle subtended at centre 1.15 radians. Area of sector of a circle =

Area of triangle = 12 × base × height.

Area of trapezium =

Calculate the areas of the following sectors of

(sum of parallel sides)(perpendicular distance between them)

The cross-sectional area of the shaft is shown by the shaded part in Fig. 22.11 (often called an annulus).

= 12 (27.4 + 8.6)(5.5) = 12 × 36 × 5.5 = 99 mm2 d⫽ 2.25 cm d ⫽ 5.45 cm

Problem 7. Find the areas of the circles having (a) a radius of 5 cm, (b) a diameter of 15 mm, (c) a circumference of 70 mm. Fig. 22.11 Area of a circle = πr 2 or

(a) Area = π r = π(5) = 25π = 78.54 cm 2

(b) Area =

2

Area of shaded part = area of large circle − area of small circle

πd 2 4 2

π d 2 π(15)2 225π = = = 176.7 mm2 4 4 4

=

πd 2 π πD2 π − = (D2 − d 2 ) = (5.452 − 2.252 ) 4 4 4 4 = 19.35 cm2

170

Basic Engineering Mathematics

Now try the following exercise

120 mm 6 mm

Exercise 81 Further problems on areas of plane figures (Answers on page 279)

30 mm

(iii)

2. A rectangular field has an area of 1.2 hectares and a length of 150 m. Find (a) its width and (b) the length of a diagonal (1 hectare = 10 000 m2 ). 3. Determine the area of each of the angle iron sections shown in Fig. 22.12.

26 cm

38 mm

1. A rectangular plate is 85 mm long and 42 mm wide. Find its area in square centimetres.

30 mm

3.5 cm

4 cm

10 cm

5.94 cm

(i)

(ii)

12 cm 16 cm

(iv)

Fig. 22.14

8. Determine the area of circles having (a) a radius of 4 cm (b) a diameter of 30 mm (c) a circumference of 200 mm. 9. An annulus has an outside diameter of 60 mm and an inside diameter of 20 mm. Determine its area. 10. If the area of a circle is 320 mm2 , find (a) its diameter, and (b) its circumference.

7 cm 30 mm

11. Calculate the areas of the following sectors of circles:

8 mm

25 mm

1 cm

10 mm

(a) radius 9 cm, angle subtended at centre 75◦ , 6 mm

2 cm 1 cm (a)

50 mm

2 cm

(c) diameter 5 cm, angle subtended at centre 2.19 radians.

(b)

Fig. 22.12

12. Determine the area of the template shown in Fig. 22.15.

4. A rectangular garden measures 40 m by 15 m. A 1 m flower border is made round the two shorter sides and one long side. A circular swimming pool of diameter 8 m is constructed in the middle of the garden. Find, correct to the nearest square metre, the area remaining. 5. The area of a trapezium is 13.5 cm2 and the perpendicular distance between its parallel sides is 3 cm. If the length of one of the parallel sides is 5.6 cm, find the length of the other parallel side. 6. Find the angles p, q, r, s and t in Fig. 22.13(a) to (c).

r 125° 75° 40°

62°

47° s

p

(b) diameter 35 mm, angle subtended at centre 48◦ 37 ,

38°

t

80 mm radius 120 mm

90 mm

Fig. 22.15

13. An archway consists of a rectangular opening topped by a semi-circular arch as shown in Fig. 22.16. Determine the area of the opening if the width is 1 m and the greatest height is 2 m.

57°

95°

q

(a)

(b)

(c)

2m

Fig. 22.13

7. Name the types of quadrilateral shown in Fig. 22.14(i) to (iv), and determine (a) the area, and (b) the perimeter of each.

1m

Fig. 22.16

Areas of plane figures

14. Calculate the area of the steel plate shown in Fig. 22.17. 25 25

171

A hexagon is a 6-sided polygon which may be divided into 6 equal triangles as shown in Fig. 22.19. The angle subtended at the centre of each triangle is 360◦/6 = 60◦ . The other two angles in the triangle add up to 120◦ and are equal to each other. Hence each of the triangles is equilateral with each angle 60◦ and each side 8 cm.

Dimensions in mm

4 cm 100

h 8 cm

25

60° 8 cm

60 140

Fig. 22.19

Fig. 22.17

Area of one triangle = 12 × base × height = 12 × 8 × h. h is calculated using Pythagoras’ theorem:

22.4 Further worked problems on areas of plane figures Problem 10. Calculate the area of a regular octagon, if each side is 5 cm and the width across the flats is 12 cm.

An octagon is an 8-sided polygon. If radii are drawn from the centre of the polygon to the vertices then 8 equal triangles are produced (see Fig. 22.18).

82 = h2 + 42 √ h = 82 − 42 = 6.928 cm

from which

Hence area of one triangle = 12 × 8 × 6.928 = 27.71 cm2 Area of hexagon = 6 × 27.71 = 166.3 cm2 Problem 12. Figure 22.20 shows a plan of a floor of a building which is to be carpeted. Calculate the area of the floor in square metres. Calculate the cost, correct to the nearest pound, of carpeting the floor with carpet costing £16.80 per m2 , assuming 30% extra carpet is required due to wastage in fitting.

12 cm

5m

2.

L

M

2m K 0.6 m

4m J

5 cm

Fig. 22.18

H

1 × base × height 2

12 1 = 15 cm2 = ×5× 2 2 Area of octagon

= 8 × 15 = 120 cm2

Problem 11. Determine the area of a regular hexagon which has sides 8 cm long.

B′

0.8 m

2m

Area of one triangle =

A

l

0.6 m

F 0.8 m G

60°

B

3m

C

E 2m

3m 30°

D 3m

Fig. 22.20

Area of floor plan = area of triangle ABC + area of semicircle + area of rectangle CGLM + area of rectangle CDEF − area of trapezium HIJK

172

Basic Engineering Mathematics

Triangle ABC is equilateral since AB = BC = 3 m and hence angle B CB = 60◦

22.5 Areas of similar shapes

sin B CB = BB /3, i.e. BB = 3 sin 60◦ = 2.598 m

The areas of similar shapes are proportional to the squares of corresponding linear dimensions. For example, Fig. 22.22 shows two squares, one of which has sides three times as long as the other.

Area of triangle ABC =

1 (AC)(BB ) = 12 (3)(2.598) 2

= 3.897 m2 Area of semicircle = 12 πr 2 = 12 π(2.5)2 = 9.817 m2 Area of CGLM = 5 × 7 = 35 m2

Area of Figure 22.22(a) = (x)(x) = x2 Area of Figure 22.22(b) = (3x)(3x) = 9x2

Area of CDEF = 0.8 × 3 = 2.4 m2

Hence Figure 22.22(b) has an area (3)2 , i.e. 9 times the area of Figure 22.22(a).

Area of HIJK = 12 (KH + IJ )(0.8) Since MC = 7 m then LG = 7 m, hence JI = 7 − 5.2 = 1.8 m Hence area of HIJK = 12 (3 + 1.8)(0.8) = 1.92 m2 Total floor area = 3.897 + 9.817 + 35 + 2.4 − 1.92 = 49.194 m2 To allow for 30% wastage, amount of carpet required = 1.3 × 49.194 = 63.95 m2 Cost of carpet at £16.80 per m2 = 63.95 × 16.80 = £1074, correct to the nearest pound.

3x x x (a)

Now try the following exercise

3x (b)

Fig. 22.22 Exercise 82 Further problems on areas of plane figures (Answers on page 279) 1. Calculate the area of a regular octagon if each side is 20 mm and the width across the flats is 48.3 mm. 2. Determine the area of a regular hexagon which has sides 25 mm. 3. A plot of land is in the shape shown in Fig. 22.21. Determine (a) its area in hectares (1 ha = 104 m2 ), and (b) the length of fencing required, to the nearest metre, to completely enclose the plot of land. 20 m

30 m

2m

20 m

Area of garage on the plan = 10 mm × 20 mm = 200 mm2 . Since the areas of similar shapes are proportional to the squares of corresponding dimensions then: True area of garage = 200 × (250)2

20 m

= 12.5 × 106 mm2

m

10 m

Problem 13. A rectangular garage is shown on a building plan having dimensions 10 mm by 20 mm. If the plan is drawn to a scale of 1 to 250, determine the true area of the garage in square metres.

=

12.5 × 106 2 m = 12.5 m2 106

15 m

30

20 m

Now try the following exercise 15 m

20 m

Exercise 83 40 m

Fig. 22.21

4. If paving slabs are produced in 250 mm × 250 mm squares, determine the number of slabs required to cover an area of 2 m2 .

Further problems on areas of similar shapes (Answers on page 279)

1. The area of a park on a map is 500 mm2 . If the scale of the map is 1 to 40 000 determine the true area of the park in hectares (1 hectare = 104 m2 ). 2. A model of a boiler is made having an overall height of 75 mm corresponding to an overall height of the actual boiler of 6 m. If the area of metal required for the model

Areas of plane figures

is 12 500 mm2 determine, in square metres, the area of metal required for the actual boiler. 3. The scale of an Ordinance Survey map is 1:2500. A circular sports field has a diameter of 8 cm on the map. Calculate its area in hectares, giving your answer correct to 3 significant figures. (1 hectare = 104 m2 )

173

6. A steel template is of the shape shown in Fig. A10.1, the circular area being removed. Determine the area of the template, in square centimetres, correct to 1 decimal place. (7) 30 mm 45 mm

Assignment 10

130 mm

This assignment covers the material contained in Chapters 21 and 22. The marks for each question are shown in brackets at the end of each question. 50 mm dia.

1. Change the following Cartesian co-ordinates into polar co-ordinates, correct to 2 decimal places, in both degrees and in radians: (a) (−2.3, 5.4)

(b) (7.6, −9.2)

(10)

2. Change the following polar co-ordinates into Cartesian co-ordinates, correct to 3 decimal places: (a) (6.5, 132◦ )

(b) (3, 3 rad)

(6)

3. A rectangular park measures 150 m by 70 m. A 2 m flower border is constructed round the two longer sides and one short side. A circular fish pond of diameter 15 m is in the centre of the park and the remainder of the park is grass. Calculate, correct to the nearest square metre, the area of (a) the fish pond, (b) the flower borders, (c) the grass. (10)

30 mm 70 mm

150 mm

Fig. A10.1

7. The area of a plot of land on a map is 400 mm2 . If the scale of the map is 1 to 50 000, determine the true area of the land in hectares (1 hectare = 104 m2 ). (3) 8. Determine the shaded area in Fig. A10.2, correct to the nearest square centimetre. (3)

2 cm

4. A swimming pool is 55 m long and 10 m wide. The perpendicular depth at the deep end is 5 m and at the shallow end is 1.5 m, the slope from one end to the other being uniform. The inside of the pool needs two coats of a protective paint before it is filled with water. Determine how many litres of paint will be needed if 1 litre covers 10 m2 . (7) 5. Find the area of an equilateral triangle of side 20.0 cm. (4)

60 mm

70 mm

m

20 c

Fig. A10.2

23 The circle

23.1 Introduction A circle is a plain figure enclosed by a curved line, every point on which is equidistant from a point within, called the centre.

(vii) A tangent to a circle is a straight line which meets the circle in one point only and does not cut the circle when produced. AC in Fig. 23.1 is a tangent to the circle since it touches the curve at point B only. If radius OB is drawn, then angle ABO is a right angle. (viii) A sector of a circle is the part of a circle between radii (for example, the portion OXY of Fig. 23.2 is a sector). If a sector is less than a semicircle it is called a minor sector, if greater than a semicircle it is called a major sector.

23.2 Properties of circles (i) The distance from the centre to the curve is called the radius, r, of the circle (see OP in Fig. 23.1).

X Y

Q

O

A

S

O

T R

P B R C

Fig. 23.1 (ii) The boundary of a circle is called the circumference, c. (iii) Any straight line passing through the centre and touching the circumference at each end is called the diameter, d (see QR in Fig. 23.1). Thus d = 2r circumference = a constant for any circle. (iv) The ratio diameter This constant is denoted by the Greek letter π (pronounced ‘pie’), where π = 3.14159, correct to 5 decimal places. Hence c/d = π or c = π d or c = 2πr (v) A semicircle is one half of the whole circle. (vi) A quadrant is one quarter of a whole circle.

Fig. 23.2 (ix) A chord of a circle is any straight line which divides the circle into two parts and is terminated at each end by the circumference. ST , in Fig. 23.2 is a chord. (x) A segment is the name given to the parts into which a circle is divided by a chord. If the segment is less than a semicircle it is called a minor segment (see shaded area in Fig. 23.2). If the segment is greater than a semicircle it is called a major segment (see the unshaded area in Fig. 23.2). (xi) An arc is a portion of the circumference of a circle. The distance SRT in Fig. 23.2 is called a minor arc and the distance SXYT is called a major arc. (xii) The angle at the centre of a circle, subtended by an arc, is double the angle at the circumference subtended by the same arc. With reference to Fig. 23.3, Angle AOC = 2 × angle ABC.

The circle

Q A

175

Now try the following exercise

B O

Exercise 84

P C

Further problems on properties of a circle (Answers on page 279)

1. Calculate the length of the circumference of a circle of radius 7.2 cm.

Fig. 23.3 (xiii) The angle in a semicircle is a right angle (see angle BQP in Fig. 23.3).

Problem 1. Find the circumference of a circle of radius 12.0 cm.

2. If the diameter of a circle is 82.6 mm, calculate the circumference of the circle. 3. Determine the radius of a circle whose circumference is 16.52 cm. 4. If the circumference of the earth is 40 000 km at the equator, calculate its diameter. 5. Calculate the length of wire in the paper clip shown in Fig. 23.5. The dimensions are in millimetres.

Circumference, c = 2 × π × radius = 2πr = 2π(12.0) = 75.40 cm

2.5 rad

Problem 2. If the diameter of a circle is 75 mm, find its circumference. 12

32

Circumference, c = π × diameter = πd = π(75) = 235.6 mm

2.5 rad 6

3 rad

Problem 3. Determine the radius of a circle if its perimeter is 112 cm. Perimeter = circumference, c = 2πr 112 c = = 17.83 cm Hence radius r = 2π 2π Problem 4. In Fig. 23.4, AB is a tangent to the circle at B. If the circle radius is 40 mm and AB = 150 mm, calculate the length AO. B r A

Fig. 23.5

23.3 Arc length and area of a sector One radian is defined as the angle subtended at the centre of a circle by an arc equal in length to the radius. With reference to Fig. 23.6, for arc length s, θ radians = s/r

or arc length,

where θ is in radians.

O r

Fig. 23.4

s u

o

A tangent to a circle is at right angles to a radius drawn from the point of contact, i.e. ABO = 90◦ . Hence, using Pythagoras’ theorem:

from which,

s = rθ

AO2 = AB2 + OB2 AO = AB2 + OB2 = 1502 + 402 = 155.2 mm

r

Fig. 23.6 When s = whole circumference (= 2πr) then θ = s/r = 2πr/r = 2π i.e.

2π radians = 360◦

or

π radians = 180◦

(1)

176

Basic Engineering Mathematics

Thus 1 rad = 180◦ /π = 57.30◦ , correct to 2 decimal places. Since π rad = 180◦ , then π/2 = 90◦ , π/3 = 60◦ , π/4 = 45◦ , and so on. θ (πr 2 ) Area of a sector = 360 when θ is in degrees 1 θ (πr 2 ) = r 2 θ (2) = 2π 2 when θ is in radians Problem 5. Convert to radians: (a) 125◦

(b) 69◦ 47

(a) Since 180◦ = π rad then 1◦ = π/180 rad, therefore π c = 2.182 radians 125◦ = 125 180 (Note that measure.)

c

means ‘circular measure’ and indicates radian

s = (5.5)(1.20) = 6.60 cm

Problem 9. Determine the diameter and circumference of a circle if an arc of length 4.75 cm subtends an angle of 0.91 radians. s 4.75 = = 5.22 cm. θ 0.91 Diameter = 2 × radius = 2 × 5.22 = 10.44 cm. Circumference, c = πd = π(10.44) = 32.80 cm.

47◦ = 69.783◦ 60 π c 69.783◦ = 69.783 = 1.218 radians 180

Problem 10. If an angle of 125◦ is subtended by an arc of a circle of radius 8.4 cm, find the length of (a) the minor arc, and (b) the major arc, correct to 3 significant figures.

Problem 6. Convert to degrees and minutes: (a) 0.749 radians (b) 3π/4 radians

Since 180◦ = π rad then 1◦ = π

rad and 125◦ = 125 180 Length of minor arc,

(a) Since π rad = 180◦ then 1 rad = 180◦ /π , therefore 180 ◦ = 42.915◦ 0.749 = 0.749 π

From equation (1), length of arc, s = rθ, where θ is in radians, hence

Since s = rθ then r =

(b) 69◦ 47 = 69

◦

Problem 8. Find the length of arc of a circle of radius 5.5 cm when the angle subtended at the centre is 1.20 radians.

s = rθ = (8.4)(125)

0.915 = (0.915 × 60) = 55 , correct to the nearest minute, hence 0.749 radians = 42◦ 55 180 ◦ (b) Since 1 rad = then π 3 = (180)◦ = 135◦ 4

3π 3π rad = 4 4

180 π

◦

Problem 7. Express in radians, in terms of π (a) 150◦ (b) 270◦ (c) 37.5◦ Since 180◦ = π rad then 1◦ = 180/π, hence π

5π rad rad = (a) 150◦ = 150 180 6 π

3π rad rad = (b) 270◦ = 270 180 2 π

5π 75π rad = rad rad = (c) 37.5◦ = 37.5 180 360 24

π

rad 180

π

= 18.3 cm 180

correct to 3 significant figures. Length of major arc = (circumference – minor arc) = 2π(8.4) − 18.3 = 34.5 cm, correct to 3 significant figures. (Alternatively, major arc = rθ = 8.4(360 − 125)(π/180) = 34.5 cm.) Problem 11. Determine the angle, in degrees and minutes, subtended at the centre of a circle of diameter 42 mm by an arc of length 36 mm. Calculate also the area of the minor sector formed. Since length of arc, s = rθ then θ = s/r diameter 42 = = 21 mm 2 2 s 36 = 1.7143 radians θ= = r 21

Radius, r = hence

1.7143 rad = 1.7143 × (180/π)◦ = 98.22◦ = 98◦ 13 = angle subtended at centre of circle.

The circle

177

From equation (2), area of sector =

1 2 1 r θ = (21)2 (1.7143) = 378 mm2 2 2

2.12 mm

B

Problem 12. A football stadium floodlight can spread its illumination over an angle of 45◦ to a distance of 55 m. Determine the maximum area that is floodlit. 1 Floodlit area = area of sector = r 2 θ 2 1 π

from equation (2) = (55)2 45 × 2 180 = 1188 m2 Problem 13. An automatic garden spray produces a spray to a distance of 1.8 m and revolves through an angle α which may be varied. If the desired spray catchment area is to be 2.5 m2 , to what should angle α be set, correct to the nearest degree.

10

mm

30 mm C

u 2

A

Fig. 23.8 Length BC = 10 mm (i.e. the radius of the circle), and AB = 30 − 10 − 2.12 = 17.88 mm from Fig. 23.7. θ 10 θ 10 −1 Hence sin = and = sin = 34◦ 2 17.88 2 17.88 and angle θ = 68◦ Now try the following exercise Exercise 85

Further problems on arc length and area of a sector (Answers on page 279)

1. Convert to radians in terms of π: (a) 30◦ (b) 75◦ (c) 225◦

1 2 1 r θ, hence 2.5 = (1.8)2 α 2 2 2.5 × 2 from which, α = = 1.5432 radians 1.82 180 ◦ 1.5432 rad = 1.5432 × = 88.42◦ π

Area of sector =

2. Convert to radians: (a) 48◦ (b) 84◦ 51

Hence angle α = 88◦ , correct to the nearest degree.

Problem 14. The angle of a tapered groove is checked using a 20 mm diameter roller as shown in Fig. 23.7. If the roller lies 2.12 mm below the top of the groove, determine the value of angle θ.

(c) 232◦ 15

3. Convert to degrees: 4π 7π 5π rad (b) rad (c) rad (a) 6 9 12 4. Convert to degrees and minutes: (a) 0.0125 rad (b) 2.69 rad (c) 7.241 rad 5. Find the length of an arc of a circle of radius 8.32 cm when the angle subtended at the centre is 2.14 radians. Calculate also the area of the minor sector formed. 6. If the angle subtended at the centre of a circle of diameter 82 mm is 1.46 rad, find the lengths of the (a) minor arc (b) major arc.

2.12 mm

20 mm 30 mm

u

Fig. 23.7 In Fig. 23.8, triangle ABC is right-angled at C (see Section 23.2(vii), page 174).

7. A pendulum of length 1.5 m swings through an angle of 10◦ in a single swing. Find, in centimetres, the length of the arc traced by the pendulum bob. 8. Determine the length of the radius and circumference of a circle if an arc length of 32.6 cm subtends an angle of 3.76 radians. 9. Determine the angle of lap, in degrees and minutes, if 180 mm of a belt drive are in contact with a pulley of diameter 250 mm. 10. Determine the number of complete revolutions a motorcycle wheel will make in travelling 2 km, if the wheel’s diameter is 85.1 cm.

178

Basic Engineering Mathematics

11. The floodlights at a sports ground spread its illumination over an angle of 40◦ to a distance of 48 m. Determine (a) the angle in radians, and (b) the maximum area that is floodlit.

23.4 The equation of a circle The simplest equation of a circle, centre at the origin, radius r, is given by:

12. Find the area swept out in 50 minutes by the minute hand of a large floral clock, if the hand is 2 m long. 13. Determine (a) the shaded area in Fig. 23.9 (b) the percentage of the whole sector that the shaded area represents.

x2 + y2 = r 2 For example, Fig. 23.12 shows a circle x2 + y2 = 9. y 3 x2 ⫹ y2 ⫽ 9

2 1 ⫺3 ⫺2 ⫺1 0 ⫺1 ⫺2 ⫺3

12 mm

0.75 rad

1

2

3

x

50 mm

Fig. 23.12 More generally, the equation of a circle, centre (a, b), radius r, is given by: Fig. 23.9 14. Determine the length of steel strip required to make the clip shown in Fig. 23.10.

(x − a)2 + (y − b)2 = r 2

(1)

Figure 23.13 shows a circle (x − 2)2 + (y − 3)2 = 4

100 mm y

125 mm rad

5

130°

4

b⫽3

100 mm

r⫽

2

2

0

2

4

x

a⫽2

Fig. 23.10 15. A 50◦ tapered hole is checked with a 40 mm diameter ball as shown in Fig. 23.11. Determine the length shown as x.

Fig. 23.13 The general equation of a circle is:

70 mm x

x2 + y2 + 2ex + 2fy + c = 0 Multiplying out the bracketed terms in equation (1) gives:

m 40 m

x2 − 2ax + a2 + y2 − 2by + b2 = r 2 Comparing this with equation (2) gives: 2e = −2a, i.e. a = −

50°

Fig. 23.11

and

2f = −2b, i.e. b = −

and

c = a2 + b2 − r 2 , i.e.

2e 2

2f 2 √

a2 + b2 − c

(2)

The circle

179

2e 2f From above a = − , b = − 2 2 √ 2 2 and r= a +b −c

Thus, for example, the equation x2 + y2 − 4x − 6y + 9 = 0 represents a circle with centre −6 −4 , b=− a=− 2 2 √ 2 i.e., at (2, 3) and radius r = 2 + 32 − 9 = 2

Hence if x2 + y2 − 4x + 6y − 3 = 0. 6 −4 = −3 = 2, b = − then a = − 2 2 √ and r = 22 + (−3)2 − (−3) = 16 = 4

Hence x2 + y2 − 4x − 6y + 9 = 0 is the circle shown in Fig. 23.13 which may be checked by multiplying out the brackets in the equation

Thus the circle has centre (2, −3) and radius 4, as shown in Fig. 23.15.

(x − 2)2 + (y − 3)2 = 4

y 4

Problem 15. Determine (a) the radius, and (b) the co-ordinates of the centre of the circle given by the equation: x2 + y2 + 8x − 2y + 8 = 0

2

⫺4

⫺2

2

4 r ⫽

⫺2

x2 + y2 + 8x − 2y + 8 = 0 is of the form shown in equation (2), −2 8 = −4, b = − =1 where a = − 2 2 √ and r = ( − 4)2 + 12 − 8 = 9 = 3

6

x

4

⫺3 ⫺4

Hence x2 + y2 + 8x − 2y + 8 − 0 represents a circle centre (−4, 1) and radius 3, as shown in Fig. 23.14. y

0

⫺8

Fig. 23.15

3

4

Now try the following exercise

r⫽

2 b⫽1

⫺8

⫺6

⫺4

⫺2

0

x

a ⫽ ⫺4

Fig. 23.14

Exercise 86

Further problems on the equation of a circle (Answers on page 280)

1. Determine (a) the radius, and (b) the co-ordinates of the centre of the circle given by the equation x2 + y2 + 8x − 2y + 8 = 0 2. Sketch the circle given by the equation

Problem 16. Sketch the circle given by the equation: x2 + y2 − 4x + 6y − 3 = 0 The equation of a circle, centre (a, b), radius r is given by: (x − a)2 + (y − b)2 = r 2 The general equation of a circle is x2 + y2 + 2ex + 2fy + c = 0

x2 + y2 − 6x + 4y − 3 = 0 3. Sketch the curve x2 + ( y − 1)2 − 25 = 0 4. Sketch the curve x=6

1−

y 2 6

24 Volumes of common solids

24.1 Volumes and surface areas of regular solids A summary of volumes and surface areas of regular solids is shown in Table 24.1. Table 24.1 (i) Rectangular prism (or cuboid)

h

l b

24.2 Worked problems on volumes and surface areas of regular solids Problem 1. A water tank is the shape of a rectangular prism having length 2 m, breadth 75 cm and height 50 cm. Determine the capacity of the tank in (a) m3 (b) cm3 (c) litres. Volume of rectangular prism = l × b × h (see Table 24.1)

Volume ⫽ l ⫻ b ⫻ h Surface area ⫽ 2(bh ⫹ hl ⫹ lb)

(ii) Cylinder

(a) Volume of tank = 2 × 0.75 × 0.5 = 0.75 m3 (b) 1 m3 = 106 cm3 , hence 0.75 m3 = 0.75 × 106 cm3

r h

= 750 000 cm3

Volume ⫽πr 2h Total surface area ⫽ 2 πrh ⫹ 2πr 2

(c) 1 litre = 1000 cm3 , hence 750 000 cm3 =

(iii) Pyramid 1

h

Volume ⫽ 3 ⫻ A ⫻ h where A ⫽ area of base and h ⫽ perpendicular height

Total surface area ⫽ (sum of areas of triangles forming sides) ⫹ (area of base)

750 000 litres = 750 litres 1000

Problem 2. Find the volume and total surface area of a cylinder of length 15 cm and diameter 8 cm.

(iv) Cone l

h r

Volume ⫽

1 3

πr 2h

Since diameter = 8 cm, then radius r = 4 cm Curved surface area ⫽ πrl Total surface area ⫽ πrl ⫹ πr 2

(v) Sphere 4

Volume ⫽ 3 πr 3 r

Volume of cylinder = πr 2 h (see Table 24.1)

Surface area ⫽ 4πr 2

Hence volume = π × 42 × 15 = 754 cm3 Total surface area (i.e. including the two ends) = 2πrh + 2πr 2 = (2 × π × 4 × 15) + (2 × π × 42 ) = 477.5 cm2

Volumes of common solids

Problem 3. Determine the volume (in cm3 ) of the shape shown in Fig. 24.1.

181

Problem 5. Determine the volume and the total surface area of the square pyramid shown in Fig. 24.3 if its perpendicular height is 12 cm.

16 mm A 12 mm

40 mm

Fig. 24.1

The solid shown in Fig. 24.1 is a triangular prism. The volume V of any prism is given by: V = Ah, where A is the cross-sectional area and h is the perpendicular height.

B

E

5 cm C

Hence volume = 12 × 16 × 12 × 40

D

= 3840 mm3 = 3.840 cm3

5 cm

Fig. 24.3 (since 1 cm3 = 1000 mm3 ) Volume of pyramid = 13 (area of base)(perpendicular height)

Problem 4. Calculate the volume and total surface area of the solid prism shown in Fig. 24.2. 11 cm

= 13 (5 × 5) × 12 = 100 cm3 The total surface area consists of a square base and 4 equal triangles. Area of triangle ADE = 12 × base × perpendicular height

4 cm

= 12 × 5 × AC

15 cm

The length AC may be calculated using Pythagoras’ theorem on triangle ABC, where AB = 12 cm, BC = 12 × 5 = 2.5 cm AC = AB2 + BC 2 = 122 + 2.52 = 12.26 cm Hence area of triangle ADE = 12 × 5 × 12.26 = 30.65 cm2 Total surface area of pyramid = (5 × 5) + 4(30.65) = 147.6 cm2

5 cm

5 cm 5 cm

Fig. 24.2

Problem 6. Determine the volume and total surface area of a cone of radius 5 cm and perpendicular height 12 cm. The cone is shown in Fig. 24.4.

The solid shown in Fig. 24.2 is a trapezoidal prism.

Volume of cone= 13 πr 2 h = 13 × π × 52 × 12 = 314.2 cm3

Volume = cross-sectional area × height

Total surface area = curved surface area + area of base

=

1 (11 + 5)4 × 15 = 32 × 15 = 480 cm3 2

Surface area = sum of two trapeziums + 4 rectangles = (2 × 32) + (5 × 15) + (11 × 15) + 2(5 × 15) = 64 + 75 + 165 + 150 = 454 cm2

= πrl + πr 2 From Fig. 24.4, slant height l may be calculated using Pythagoras’ theorem l = 122 + 52 = 13 cm Hence total surface area = (π × 5 × 13) + (π × 52 ) = 282.7 cm2

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Basic Engineering Mathematics

9. A square pyramid has a perpendicular height of 4 cm. If a side of the base is 2.4 cm long find the volume and total surface area of the pyramid. l

10. A sphere has a diameter of 6 cm. Determine its volume and surface area.

h⫽ 12 cm

11. Find the total surface area of a hemisphere of diameter 50 mm. r ⫽ 5 cm

12. How long will it take a tap dripping at a rate of 800 mm3 /s to fill a 3-litre can?

Fig. 24.4

Problem 7. Find the volume and surface area of a sphere of diameter 8 cm. Since diameter = 8 cm, then radius, r = 4 cm. Volume of sphere = 43 πr 3 = 43 × π × 43 = 268.1 cm3

24.3 Further worked problems on volumes and surface areas of regular solids Problem 8. A wooden section is shown in Fig. 24.5. Find (a) its volume (in m3 ), and (b) its total surface area.

Surface area of sphere = 4πr 2 = 4 × π × 42 = 201.1 cm2

r⫽

Now try the following exercise

3m

8m

m

r 12 cm

Exercise 87 Further problems on volumes and surface areas of regular solids (Answers on page 280) 1. A rectangular block of metal has dimensions of 40 mm by 25 mm by 15 mm. Determine its volume. Find also its mass if the metal has a density of 9 g/cm3 .

Fig. 24.5 (a) The section of wood is a prism whose end comprises a rectangle and a semicircle. Since the radius of the semicircle is 8 cm, the diameter is 16 cm.

2. Determine the maximum capacity, in litres, of a fish tank measuring 50 cm by 40 cm by 2.5 m (1 litre = 1000 cm3 ).

Hence the rectangle has dimensions 12 cm by 16 cm.

3. Determine how many cubic metres of concrete are required for a 120 m long path, 150 mm wide and 80 mm deep.

1 Area of end = (12 × 16) + π82 = 292.5 cm2 2 Volume of wooden section

4. Calculate the volume of a metal tube whose outside diameter is 8 cm and whose inside diameter is 6 cm, if the length of the tube is 4 m.

= area of end × perpendicular height

3

5. The volume of a cylinder is 400 cm . If its radius is 5.20 cm, find its height. Determine also its curved surface area. 6. If a cone has a diameter of 80 mm and a perpendicular height of 120 mm calculate its volume in cm3 and its curved surface area. 7. A cylinder is cast from a rectangular piece of alloy 5 cm by 7 cm by 12 cm. If the length of the cylinder is to be 60 cm, find its diameter. 8. Find the volume and the total surface area of a regular hexagonal bar of metal of length 3 m if each side of the hexagon is 6 cm.

= 292.5 × 300 = 87750 cm3 =

87750 m3 106

= 0.08775 m3 (b) The total surface area comprises the two ends (each of area 292.5 cm2 ), three rectangles and a curved surface (which is half a cylinder), hence total surface area = (2 × 292.5) + 2(12 × 300) + (16 × 300) 1 + (2π × 8 × 300) 2 = 585 + 7200 + 4800 + 2400π = 20125 cm2

or 2.0125 m2

Volumes of common solids

Problem 9. A pyramid has a rectangular base 3.60 cm by 5.40 cm. Determine the volume and total surface area of the pyramid if each of its sloping edges is 15.0 cm.

183

Similarly, if H is the mid-point of AB, then FH =

15.02 − 2.702 = 14.75 cm,

hence area of triangle ABF (which equals triangle CDF) The pyramid is shown in Fig. 24.6. To calculate the volume of the pyramid the perpendicular height EF is required. Diagonal BD is calculated using Pythagoras’ theorem, i.e. BD = [3.602 + 5.402 ] = 6.490 cm

= 12 (5.40)(14.75) = 39.83 cm2 Total surface area of pyramid = 2(26.80) + 2(39.83) + (3.60)(5.40) = 53.60 + 79.66 + 19.44 = 152.7 cm2

F

1 Volume of hemisphere = (volume of sphere) 2 2 5.0 3 2 = πr 3 = π = 32.7 cm2 3 3 2 Total surface area

cm 15.0 cm 15.0

15.0 cm

15.0

cm

Problem 10. Calculate the volume and total surface area of a hemisphere of diameter 5.0 cm.

= curved surface area + area of circle

C

1 = (surface area of sphere) + π r 2 2 1 = (4πr 2 ) + πr 2 2 5.0 2 = 2πr 2 + πr 2 = 3πr 2 = 3π = 58.9 cm2 2

D

3.60 c

m

E G

B

A

H m 5.40 c

Problem 11. A rectangular piece of metal having dimensions 4 cm by 3 cm by 12 cm is melted down and recast into a pyramid having a rectangular base measuring 2.5 cm by 5 cm. Calculate the perpendicular height of the pyramid.

Fig. 24.6 6.490 1 = 3.245 cm. Hence EB = BD = 2 2 Using Pythagoras’ theorem on triangle BEF gives

from which,

Volume of rectangular prism of metal = 4 × 3 × 12 = 144 cm3

BF 2 = EB2 + EF 2 EF = (BF 2 − EB2 ) √ = 15.02 − 3.2452 = 14.64 cm.

Volume of pyramid = 13 (area of base)(perpendicular height) Assuming no waste of metal,

Volume of pyramid

144 =

= 13 (area of base)(perpendicular height) = 13 (3.60 × 5.40)(14.64) = 94.87 cm3 Area of triangle ADF (which equals triangle BCF) = where G is the midpoint of AD. Using Pythagoras’ theorem on triangle FGA gives

1 (AD)(FG), 2

FG =

[15.02 − 1.802 ] = 14.89 cm

Hence area of triangle ADF = 12 (3.60)(14.89) = 26.80 cm2

i.e.

perpendicular height =

1 (2.5 × 5)(height) 3 144 × 3 = 34.56 cm 2.5 × 5

Problem 12. A rivet consists of a cylindrical head, of diameter 1 cm and depth 2 mm, and a shaft of diameter 2 mm and length 1.5 cm. Determine the volume of metal in 2000 such rivets.

184

Basic Engineering Mathematics

1 cm = 0.5 cm and 2 height of cylindrical head = 2 mm = 0.2 cm Hence, volume of cylindrical head

Radius of cylindrical head =

= π r 2 h = π (0.5)2 (0.2) = 0.1571 cm3 Volume of cylindrical shaft

0.2 = πr h = π 2 2

2 (1.5) = 0.0471 cm3

Total volume of 1 rivet = 0.1571 + 0.0471 = 0.2042 cm3 Volume of metal in 2000 such rivets = 2000 × 0.2042 = 408.4 cm3

i.e. height of conical portion, h=

496.8 − 144 = 29.4 cm 12

Problem 14. A block of copper having a mass of 50 kg is drawn out to make 500 m of wire of uniform cross-section. Given that the density of copper is 8.91 g/cm3 , calculate (a) the volume of copper, (b) the cross-sectional area of the wire, and (c) the diameter of the cross-section of the wire. (a) A density of 8.91 g/cm3 means that 8.91 g of copper has a volume of 1 cm3 , or 1 g of copper has a volume of (1/8.91) cm3 Hence 50 kg, i.e. 50 000 g, has a volume

Problem 13. A solid metal cylinder of radius 6 cm and height 15 cm is melted down and recast into a shape comprising a hemisphere surmounted by a cone. Assuming that 8% of the metal is wasted in the process, determine the height of the conical portion, if its diameter is to be 12 cm. Volume of cylinder = πr 2 h = π × 62 × 15 = 540π cm3 If 8% of metal is lost then 92% of 540π gives the volume of the new shape (shown in Fig. 24.7).

50 000 cm3 = 5612 cm3 8.91 (b) Volume of wire = area of circular cross-section × length of wire. Hence 5612 cm3 = area × (500 × 100 cm), 5612 from which, area = cm2 = 0.1122 cm2 500 × 100 (c) Area of circle = πr 2 or from which d=

π d2 πd 2 , hence 0.1122 = 4 4

4 × 0.1122 π

= 0.3780 cm

h

i.e. diameter of cross-section is 3.780 mm. r 12 cm

Fig. 24.7

Problem 15. A boiler consists of a cylindrical section of length 8 m and diameter 6 m, on one end of which is surmounted a hemispherical section of diameter 6 m, and on the other end a conical section of height 4 m and base diameter 6 m. Calculate the volume of the boiler and the total surface area.

Hence the volume of (hemisphere + cone) = 0.92 × 540π cm3 , 1 1 4 3 π r + π r 2 h = 0.92 × 540π i.e. 2 3 3

The boiler is shown in Fig. 24.8.

Dividing throughout by π gives:

Volume of hemisphere,

2 3 1 2 r + r h = 0.92 × 540 3 3 Since the diameter of the new shape is to be 12 cm, then radius r = 6 cm, 2 1 hence (6)3 + (6)2 h = 0.92 × 540 3 3 144 + 12h = 496.8

P = 23 πr 3 =

2 3

× π × 33 = 18π m3

Volume of cylinder, Q = πr 2 h = π × 32 × 8 = 72π m3 Volume of cone, R = 13 πr 2 h =

1 3

× π × 32 × 4 = 12π m3

Volumes of common solids

the surface area of material needed to make the marquee assuming 12% of the material is wasted in the process.

P

5. Determine (a) the volume and (b) the total surface area of the following solids:

6m 8m

(i) a cone of radius 8.0 cm and perpendicular height 10 cm

Q A

(ii) a sphere of diameter 7.0 cm

3m B R

4m

(iii) a hemisphere of radius 3.0 cm

I

(iv) a 2.5 cm by 2.5 cm square pyramid of perpendicular height 5.0 cm

C

(v) a 4.0 cm by 6.0 cm rectangular pyramid of perpendicular height 12.0 cm

Fig. 24.8

(vi) a 4.2 cm by 4.2 cm square pyramid whose sloping edges are each 15.0 cm

Total volume of boiler = 18π + 72π + 12π

(vii) a pyramid having an octagonal base of side 5.0 cm and perpendicular height 20 cm.

= 102π = 320.4 m3 Surface area of hemisphere, P=

185

1 (4πr 2 ) 2

= 2 × π × 3 = 18π m 2

2

Curved surface area of cylinder, Q = 2π rh = 2 × π × 3 × 8 = 48π m2 The slant height of the cone, l, is obtained by Pythagoras’theorem on triangle ABC, i.e. l = (42 + 32 ) = 5 Curved surface area of cone, R = πrl = π × 3 × 5 = 15π m2 Total surface area of boiler = 18π + 48π + 15π = 81π = 254.5 m2

Now try the following exercise Exercise 88 Further problems on volumes and surface areas of regular solids (Answers on page 280)

6. The volume of a sphere is 325 cm3 . Determine its diameter. 7. A metal sphere weighing 24 kg is melted down and recast into a solid cone of base radius 8.0 cm. If the density of the metal is 8000 kg/m3 determine (a) the diameter of the metal sphere and (b) the perpendicular height of the cone, assuming that 15% of the metal is lost in the process. 8. Find the volume of a regular hexagonal pyramid if the perpendicular height is 16.0 cm and the side of base is 3.0 cm. 9. A buoy consists of a hemisphere surmounted by a cone. The diameter of the cone and hemisphere is 2.5 m and the slant height of the cone is 4.0 m. Determine the volume and surface area of the buoy. 10. A petrol container is in the form of a central cylindrical portion 5.0 m long with a hemispherical section surmounted on each end. If the diameters of the hemisphere and cylinder are both 1.2 m determine the capacity of the tank in litres (1 litre = 1000 cm3 ). 11. Figure 24.9 shows a metal rod section. Determine its volume and total surface area.

1. Determine the mass of a hemispherical copper container whose external and internal radii are 12 cm and 10 cm. Assuming that 1 cm3 of copper weighs 8.9 g. 2. If the volume of a sphere is 566 cm3 , find its radius. 3. A metal plumb bob comprises a hemisphere surmounted by a cone. If the diameter of the hemisphere and cone are each 4 cm and the total length is 5 cm, find its total volume. 4. A marquee is in the form of a cylinder surmounted by a cone. The total height is 6 m and the cylindrical portion has a height of 3.5 m, with a diameter of 15 m. Calculate

1.00 cm radius

2.50 cm

Fig. 24.9

1.00 m

186

Basic Engineering Mathematics

10

0

12. Find the volume (in cm3 ) of the die-casting shown in Fig. 24.10. The dimensions are in millimetres.

60 30 rad

50

25

24.4 Volumes and surface areas of frusta of pyramids and cones The frustum of a pyramid or cone is the portion remaining when a part containing the vertex is cut off by a plane parallel to the base. The volume of a frustum of a pyramid or cone is given by the volume of the whole pyramid or cone minus the volume of the small pyramid or cone cut off. The surface area of the sides of a frustum of a pyramid or cone is given by the surface area of the whole pyramid or cone minus the surface area of the small pyramid or cone cut off. This gives the lateral surface area of the frustum. If the total surface area of the frustum is required then the surface area of the two parallel ends are added to the lateral surface area. There is an alternative method for finding the volume and surface area of a frustum of a cone. With reference to Fig. 24.12: Volume = 13 πh(R2 + Rr + r 2 )

Fig. 24.10

Curved surface area = πl(R+ r) Total surface area = πl(R + r) + πr 2 + πR2

13. The cross-section of part of a circular ventilation shaft is shown in Fig. 24.11, ends AB and CD being open. Calculate (a) the volume of the air, correct to the nearest litre, contained in the part of the system shown, neglecting the sheet metal thickness, (given 1 litre = 1000 cm3 ), (b) the cross-sectional area of the sheet metal used to make the system, in square metres, and (c) the cost of the sheet metal if the material costs £11.50 per square metre, assuming that 25% extra metal is required due to wastage.

r I

h

R

Fig. 24.12 2m A

Problem 16. Determine the volume of a frustum of a cone if the diameter of the ends are 6.0 cm and 4.0 cm and its perpendicular height is 3.6 cm.

500 mm B 1.5 m

Method 1 A section through the vertex of a complete cone is shown in Fig. 24.13. Using similar triangles AP DR = DP BR

1.5 m

AP 3.6 = 2.0 1.0

from which

AP =

D

C 800 mm

Fig. 24.11

Hence

(2.0)(3.6) = 7.2 cm 1.0

The height of the large cone = 3.6 + 7.2 = 10.8 cm.

Volumes of common solids

187

From Fig. 24.13, using Pythagoras’ theorem: AB2 = AQ2 + BQ2 , from which AB = [10.82 + 3.02 ] = 11.21 cm

A

and AD2 = AP 2 + DP 2 , from which AD = [7.22 + 2.02 ] = 7.47 cm Curved surface area of large cone = πrl = π(BQ)(AB) = π(3.0)(11.21) = 105.65 cm2 4.0 cm

and curved surface area of small cone

E

D

P

= π(DP)(AD) = π(2.0)(7.47) = 46.94 cm2

2.0 cm 3.6 cm

Hence, curved surface area of frustum = 105.65 − 46.94 = 58.71 cm2 Total surface area of frustum

R

B

C Q

= 58.71 + π(2.0)2 + π(3.0)2

1.0 cm

= 58.71 + 12.57 + 28.27 = 99.6 cm2

3.0 cm 6.0 cm

Method 2

Fig. 24.13

From page 186, total surface area of frustum = πl(R + r) + πr 2 + πR2 ,

Volume of frustum of cone = volume of large cone − volume of small cone cut off =

= curved surface area + area of two circular ends

1 π (3.0)2 (10.8) − 13 π(2.0)2 (7.2) 3

= 101.79 − 30.16 = 71.6 cm3

where l = BD = 11.21 − 7.47 = 3.74 cm, R = 3.0 cm and r = 2.0 cm. Hence total surface area of frustum = π(3.74)(3.0 + 2.0) + π (2.0)2 + π(3.0)2 = 99.6 cm2

Method 2 From above, volume of the frustum of a cone = 13 π h(R2 + Rr + r 2 ), r = 2.0 cm

where R = 3.0 cm,

and h = 3.6 cm

Hence volume of frustum = 13 π (3.6) (3.0)2 + (3.0)(2.0) + (2.0)2 = 13 π (3.6)(19.0) = 71.6 cm3 Problem 17. Find the total surface area of the frustum of the cone in Problem 16.

Method 1 Curved surface area of frustum = curved surface area of large cone − curved surface area of small cone cut off.

Problem 18. A storage hopper is in the shape of a frustum of a pyramid. Determine its volume if the ends of the frustum are squares of sides 8.0 m and 4.6 m, respectively, and the perpendicular height between its ends is 3.6 m.

The frustum is shown shaded in Fig. 24.14(a) as part of a complete pyramid. A section perpendicular to the base through the vertex is shown in Fig. 24.14(b). CG BH = BG AH (2.3)(3.6) BH = 4.87 m = Height CG = BG AH 1.7 By similar triangles:

Height of complete pyramid = 3.6 + 4.87 = 8.47 m 1 Volume of large pyramid = (8.0)2 (8.47) = 180.69 m3 3

188

Basic Engineering Mathematics

Problem 20. A lampshade is in the shape of a frustum of a cone. The vertical height of the shade is 25.0 cm and the diameters of the ends are 20.0 cm and 10.0 cm, respectively. Determine the area of the material needed to form the lampshade, correct to 3 significant figures.

C

4.6 cm

8.0 m

4.6 cm

G

B

D

2.3 m 2.3 m H

A

The curved surface area of a frustum of a cone

F E

1.7 m 2.3 m

8.0 m (a)

= πl(R + r) from page 186.

3.6 m

Since the diameters of the ends of the frustum are 20.0 cm and 10.0 cm, then from Fig. 24.16,

4.0 m

(b)

and

r = 5.0 cm, R = 10.0 cm l = [25.02 + 5.02 ] = 25.50 cm,

from Pythagoras’ theorem.

Fig. 24.14

r ⫽ 5.0 cm

Volume of small pyramid cut off =

1 (4.6)2 (4.87) = 34.35 m3 3 h ⫽ 25.0 cm

Hence volume of storage hopper = 180.69 − 34.35 = 146.3 m3 Problem 19. Determine the lateral surface area of the storage hopper in Problem 18.

I

5.0 cm

The lateral surface area of the storage hopper consists of four equal trapeziums. 4.6 m R

Fig. 24.16

Q

4.6 m

S

P 0

R ⫽ 10.0 cm

T 8.0 m

U 8.0 m

Hence curved surface area = π(25.50)(10.0 + 5.0) = 1201.7 cm2 , i.e. the area of material needed to form the lampshade is 1200 cm2 , correct to 3 significant figures. Problem 21. A cooling tower is in the form of a cylinder surmounted by a frustum of a cone as shown in Fig. 24.17. Determine the volume of air space in the tower if 40% of the space is used for pipes and other structures. 12.0 m

Fig. 24.15 From Fig. 24.15, area of trapezium PRSU

30.0 m

= 12 (PR + SU )(QT ) 12.0 m

OT = 1.7 m (same as AH in Fig. 24.14(b) and OQ = 3.6 m. By Pythagoras’ theorem, QT = (OQ2 + OT 2 ) = [3.62 + 1.72 ] = 3.98 m 25.0 m

Area of trapezium PRSU = 12 (4.6 + 8.0)(3.98) = 25.07 m2 Lateral surface area of hopper = 4(25.07) = 100.3 m2

Fig. 24.17

Volumes of common solids

Volume of cylindrical portion 25.0 2 = πr 2 h = π (12.0) = 5890 m3 2 Volume of frustum of cone =

1 π h(R2 + Rr + r 2 ) 3

where h = 30.0 − 12.0 = 18.0 m, R = 25.0/2 = 12.5 m and r = 12.0/2 = 6.0 m. Hence volume of frustum of cone 1 = π (18.0) (12.5)2 + (12.5)(6.0) + (6.0)2 = 5038 m3 3

189

7. A cylindrical tank of diameter 2.0 m and perpendicular height 3.0 m is to be replaced by a tank of the same capacity but in the form of a frustum of a cone. If the diameters of the ends of the frustum are 1.0 m and 2.0 m, respectively, determine the vertical height required.

24.5 Volumes of similar shapes The volumes of similar bodies are proportional to the cubes of corresponding linear dimensions. For example, Fig. 24.18 shows two cubes, one of which has sides three times as long as those of the other.

Total volume of cooling tower = 5890 + 5038 = 10 928 m3 . If 40% of space is occupied then volume of air space = 0.6 × 10 928 = 6557 m3 3x

Now try the following exercise 3x

x

Exercise 89 Further problems on volumes and surface areas of frustra of pyramids and cones (Answers on page 280) 1. The radii of the faces of a frustum of a cone are 2.0 cm and 4.0 cm and the thickness of the frustum is 5.0 cm. Determine its volume and total surface area. 2. A frustum of a pyramid has square ends, the squares having sides 9.0 cm and 5.0 cm, respectively. Calculate the volume and total surface area of the frustum if the perpendicular distance between its ends is 8.0 cm. 3. A cooling tower is in the form of a frustum of a cone. The base has a diameter of 32.0 m, the top has a diameter of 14.0 m and the vertical height is 24.0 m. Calculate the volume of the tower and the curved surface area. 4. A loudspeaker diaphragm is in the form of a frustum of a cone. If the end diameters are 28.0 cm and 6.00 cm and the vertical distance between the ends is 30.0 cm, find the area of material needed to cover the curved surface of the speaker. 5. A rectangular prism of metal having dimensions 4.3 cm by 7.2 cm by 12.4 cm is melted down and recast into a frustum of a square pyramid, 10% of the metal being lost in the process. If the ends of the frustum are squares of side 3 cm and 8 cm respectively, find the thickness of the frustum. 6. Determine the volume and total surface area of a bucket consisting of an inverted frustum of a cone, of slant height 36.0 cm and end diameters 55.0 cm and 35.0 cm.

x x

3x

(a)

(b)

Fig. 24.18

Volume of Fig. 24.18(a) = (x)(x)(x) = x3 Volume of Fig. 24.18(b) = (3x)(3x)(3x) = 27x3 Hence Fig. 24.18(b) has a volume (3)3 , i.e. 27 times the volume of Fig. 24.18(a). Problem 22. A car has a mass of 1000 kg. A model of the car is made to a scale of 1 to 50. Determine the mass of the model if the car and its model are made of the same material. Volume of model = Volume of car

1 50

3

since the volume of similar bodies are proportional to the cube of corresponding dimensions. Mass = density × volume, and since both car and model are made of the same material then: Mass of model = Mass of car

1 50

3

190

Basic Engineering Mathematics

Hence mass of mode = (mass of car) =

1 50

1000 = 0.008 kg 503

3 the nearest revolution) when travelling 3 miles (assume 1 mile = 1.6 km). (5) or 8 g 5. The equation of a circle is: x2 + y2 + 12x − 4y + 4 = 0

Now try the following exercise Exercise 90 Further problems on volumes of similar shapes (Answers on page 280) 1. The diameter of two spherical bearings are in the ratio 2:5. What is the ratio of their volumes? 2. An engineering component has a mass of 400 g. If each of its dimensions are reduced by 30% determine its new mass.

Determine (a) the diameter of the circle, and (b) the co-ordinates of the centre of the circle. (5) 6. Determine the volume (in cubic metres) and the total surface area (in square metres) of a solid metal cone of base radius 0.5 m and perpendicular height 1.20 m. Give answers correct to 2 decimal places. (5) 7. A rectangular storage container has dimensions 3.2 m by 90 cm by 60 cm. Determine its volume in (a) m3 (b) cm3

(4)

8. Calculate

Assignment 11 This assignment covers the material contained in Chapters 23 and 24. The marks for each question are shown in brackets at the end of each question. 1. Determine the diameter of a circle whose circumference is 178.4 cm. (2) 2. Convert (a) 125◦ 47 to radians (b) 1.724 radians to degrees and minutes (4)

(a) the volume, and (b) the total surface area of a 10 cm by 15 cm rectangular pyramid of height 20 cm. (7) 9. A water container is of the form of a central cylindrical part 3.0 m long and diameter 1.0 m, with a hemispherical section surmounted at each end as shown in Fig. A11.2. Determine the maximum capacity of the container, correct to the nearest litre. (1 litre = 1000 cm3 ) (5) 3.0 m

3. Calculate the length of metal strip needed to make the clip shown in Fig. A11.1. (6) 30 mm rad

75 mm

1.0 m

Fig. A11.2 15 mm rad

70 mm

15 mm rad

70 mm

Fig. A11.1 4. A lorry has wheels of radius 50 cm. Calculate the number of complete revolutions a wheel makes (correct to

10. Find the total surface area of a bucket consisting of an inverted frustum of a cone, of slant height 35.0 cm and end diameters 60.0 cm and 40.0 cm. (4) 11. A boat has a mass of 20 000 kg. A model of the boat is made to a scale of 1 to 80. If the model is made of the same material as the boat, determine the mass of the model (in grams). (3)

25 Irregular areas and volumes and mean values of waveforms

25.1 Areas of irregular figures Areas of irregular plane surfaces may be approximately determined by using (a) a planimeter, (b) the trapezoidal rule, (c) the mid-ordinate rule, and (d) Simpson’s rule. Such methods may be used, for example, by engineers estimating areas of indicator diagrams of steam engines, surveyors estimating areas of plots of land or naval architects estimating areas of water planes or transverse sections of ships.

In general, the trapezoidal rule states:    sum of width of 1 first + last + remaining Area = interval 2 ordinate ordinates

(c) Mid-ordinate rule To determine the area ABCD of Fig. 25.2:

(a) A planimeter is an instrument for directly measuring small areas bounded by an irregular curve. (b) Trapezoidal rule To determine the areas PQRS in Fig. 25.1:

B y1 y2 y3 y4 y5 y6 A

D d

Q y1 y2 y3 y4 y5 y6 P

d

d

d

d

d

d

R y7

C

d

d

d

d

d

Fig. 25.2

S

Fig. 25.1

(i) Divide base PS into any number of equal intervals, each of width d (the greater the number of intervals, the greater the accuracy). (ii) Accurately measure ordinates y1 , y2 , y3 , etc. (iii) Area PQRS y1 + y 7 + y2 + y3 + y4 + y5 + y6 =d 2

(i) Divide base AD into any number of equal intervals, each of width d (the greater the number of intervals, the greater the accuracy). (ii) Erect ordinates in the middle of each interval (shown by broken lines in Fig. 25.2). (iii) Accurately measure ordinates y1 , y2 , y3 , etc. (iv) Area ABCD = d( y1 + y2 + y3 + y4 + y5 + y6 ). In general, the mid-ordinate rule states: Area = (width of interval)(sum of mid-ordinates)

192

Basic Engineering Mathematics

(d) Simpson’s rule To determine the area PQRS of Fig. 25.1: (i) Divide base PS into an even number of intervals, each of width d (the greater the number of intervals, the greater the accuracy).

The time base is divided into 6 strips each of width 1 s, and the length of the ordinates measured. Thus 0 + 24.0 area = (1) + 2.5 + 5.5 + 8.75 2 + 12.5 + 17.5 = 58.75 m

(ii) Accurately measure ordinates y1 , y2 , y3 , etc. (iii) Area PQRS =

(b) Mid-ordinate rule (see para. (c) above).

d [( y1 + y7 ) + 4( y2 + y4 + y6 ) + 2( y3 + y5 )] 3

In general, Simpson’s rule states:

The time base is divided into 6 strips each of width 1 second. Mid-ordinates are erected as shown in Fig. 25.3 by the broken lines. The length of each mid-ordinate is measured. Thus area = (1)[1.25 + 4.0 + 7.0 + 10.75 + 15.0 + 20.25] = 58.25 m

1 Area = (width of interval) [(first + last ordinate) 3 + 4(sum of even ordinates) + 2(sum of remaining odd ordinates)]

(c) Simpson’s rule (see para. (d) above). The time base is divided into 6 strips each of width 1 s, and the length of the ordinates measured. Thus area = 13 (1)[(0 + 24.0) + 4(2.5 + 8.75 + 17.5) + 2(5.5 + 12.5)] = 58.33 m

Problem 1. A car starts from rest and its speed is measured every second for 6 s: Time t (s) Speed v (m/s)

0 0

1 2.5

2 5.5

3 8.75

4 12.5

5 17.5

6 24.0

Determine the distance travelled in 6 seconds (i.e. the area under the v/t graph), by (a) the trapezoidal rule, (b) the mid-ordinate rule, and (c) Simpson’s rule.

Problem 2. A river is 15 m wide. Soundings of the depth are made at equal intervals of 3 m across the river and are as shown below. Depth (m)

0

2.2

3.3

4.5

4.2

2.4

0

Calculate the cross-sectional area of the flow of water at this point using Simpson’s rule.

A graph of speed/time is shown in Fig. 25.3. (a) Trapezoidal rule (see para. (b) above).

From para. (d) above, Area = 13 (3)[(0 + 0) + 4(2.2 + 4.5 + 2.4) + 2(3.3 + 4.2)] = (1)[0 + 36.4 + 15] = 51.4 m2

30 Graph of speed/time 25 Speed (m/s)

Now try the following exercise 20

Exercise 91 15

0

Fig. 25.3

1

2

3 4 5 Time (seconds)

6

24.0

20.25

15.0 17.5

5.5 7.0

2.5 4.0

1.25

5

8.75 10.75 12.5

10

Further problems on areas of irregular figures (Answers on page 280)

1. Plot a graph of y = 3x − x2 by completing a table of values of y from x = 0 to x = 3. Determine the area enclosed by the curve, the x-axis and ordinate x = 0 and x = 3 by (a) the trapezoidal rule, (b) the mid-ordinate rule and (c) by Simpson’s rule. 2. Plot the graph of y = 2x2 + 3 between x = 0 and x = 4. Estimate the area enclosed by the curve, the ordinates x = 0 and x = 4, and the x-axis by an approximate method.

Irregular areas and volumes and mean values of waveforms

3. The velocity of a car at one second intervals is given in the following table: time t (s) 0 1 velocity v (m/s) 0 2.0

2 4.5

3 8.0

4 5 6 14.0 21.0 29.0

Determine the distance travelled in 6 seconds (i.e. the area under the v/t graph) using an approximate method. 4. The shape of a piece of land is shown in Fig. 25.4. To estimate the area of the land, a surveyor takes measurements at intervals of 50 m, perpendicular to the straight portion with the results shown (the dimensions being in metres). Estimate the area of the land in hectares (1 ha = 104 m2 ).

193

Problem 3. A tree trunk is 12 m in length and has a varying cross-section. The cross-sectional areas at intervals of 2 m measured from one end are: 0.52,

0.55,

0.59,

0.63,

0.72,

0.84,

0.97 m2

Estimate the volume of the tree trunk.

A sketch of the tree trunk is similar to that shown in Fig. 25.5, where d = 2 m, A1 = 0.52 m2 , A2 = 0.55 m2 , and so on. Using Simpson’s rule for volumes gives: Volume = 23 [(0.52 + 0.97) + 4(0.55 + 0.63 + 0.84) + 2(0.59 + 0.72)] =

2 [1.49 3

+ 8.08 + 2.62] = 8.13 m3

Problem 4. The areas of seven horizontal cross sections of a water reservoir at intervals of 10 m are:

140 160 200 190 180 130

210, 50 50 50 50 50 50

250,

320,

350,

290,

230,

170 m2

Calculate the capacity of the reservoir in litres.

Fig. 25.4 Using Simpson’s rule for volumes gives: 5. The deck of a ship is 35 m long. At equal intervals of 5 m the width is given by the following table: Width (m)

0

2.8

5.2

6.5

5.8

4.1

3.0

Volume =

2.3

Estimate the area of the deck.

=

25.2 Volumes of irregular solids

10 [(210 + 170) + 4(250 + 350 + 230) 3 + 2(320 + 290)] 10 [380 + 3320 + 1220] = 16400 m3 3

16400 m = 16400 × 106 cm3 Since 1 litre = 1000cm3 , capacity of reservoir

If the cross-sectional areas A1 , A2 , A3 , . . . of an irregular solid bounded by two parallel planes are known at equal intervals of width d (as shown in Fig. 25.5), then by Simpson’s rule:

=

16400 × 106 litres 1000

= 16400000 = 1.64 × 107 litres Volume, V =

d [(A1 + A7 ) + 4(A2 + A4 + A6 ) 3 + 2(A3 + A5 )]

Now try the following exercise Exercise 92

A1

A2

A3

A4

A5 A6

A7

1. The areas of equidistantly spaced sections of the underwater form of a small boat are as follows: 1.76,

d

Fig. 25.5

d

d

d

d

Further problems on volumes of irregular solids (Answers on page 280)

2.78,

3.10,

3.12,

2.61,

1.24,

0.85 m2

d

Determine the underwater volume if the sections are 3 m apart.

194

Basic Engineering Mathematics

V Vm

2. To estimate the amount of earth to be removed when constructing a cutting the cross-sectional area at intervals of 8 m were estimated as follows:

V Vm

0

0,

2.8,

3.7,

4.5,

4.1,

2.6,

2

4

6

8

10

Circumference (m) 2.80 3.25 3.94 4.32 5.16

t

0

t (c)

12

5.82

(b)

V Vm

3. The circumference of a 12 m long log of timber of varying circular cross-section is measured at intervals of 2 m along its length and the results are:

0

0

(a)

Estimate the volume of earth to be excavated.

Distance from one end (m)

t

0 m3

Fig. 25.7

6.36

Estimate the volume of the timber in cubic metres.

area under curve length of base, b

20

0

1

2

3

4 t (ms)

⫺20

3 2 1

0 ⫺1 ⫺2 ⫺3

(a) Voltage (V)

y=

Current (A)

The mean or average value, y, of the waveform shown in Fig. 25.6 is given by:

Voltage (V)

25.3 The mean or average value of a waveform

Problem 5. Determine the average values over half a cycle of the periodic waveforms shown in Fig. 25.8.

y

2 3

4

5 6 t (s)

(b)

10

0

y1 y2 y3 y4 y5 y6 y7

1

2

4 6

8 t (ms)

⫺10 (c)

d

d

d

d b

d

d

d

Fig. 25.6 If the mid-ordinate rule is used to find the area under the curve, then: y=

sum of mid-ordinates number of mid-ordinates y1 + y 2 + y 3 + y 4 + y 5 + y 6 + y 7 = for Fig. 25.6 7

For a sine wave, the mean or average value: (i) over one complete cycle is zero (see Fig. 25.7(a)), (ii) over half a cycle is 0.637 × maximum value, or 2/π × maximum value, (iii) of a full-wave rectified waveform (see Fig. 25.7(b)) is 0.637 × maximum value, (iv) of a half-wave rectified waveform (see Fig. 25.7(c)) is 0.318 × maximum value, or 1/π × maximum value.

Fig. 25.8 (a) Area under triangular waveform (a) for a half cycle is given by: 1 Area = (base)(perpendicular height) 2 1 = (2 × 10−3 )(20) = 20 × 10−3 Vs 2 area under curve length of base 20 × 10−3 Vs = = 10 V 2 × 10−3 s

Average value of waveform =

(b) Area under waveform (b) for a half cycle = (1 × 1) + (3 × 2) = 7 As area under curve Average value of waveform = length of base 7 As = = 2.33A 3s

Irregular areas and volumes and mean values of waveforms

(c) A half cycle of the voltage waveform (c) is completed in 4 ms. 1 Area under curve = {(3 − 1)10−3 }(10) = 10 × 10−3 Vs 2 Average value of waveform = =

area under curve length of base 10 × 10−3 Vs = 2.5V 4 × 10−3 s

Time (h)

0

1

2

3

4

5

6

Power (kW)

0

14

29

51

45

23

0

Plot a graph of power against time and, by using the midordinate rule, determine (a) the area under the curve and (b) the average value of the power.

The graph of power/time is shown in Fig. 25.10. Graph of power/time 50

5

40 0

Current (A)

Problem 7. The power used in a manufacturing process during a 6 hour period is recorded at intervals of 1 hour as shown below.

4

8 12 16 20 24 28 t (ms)

2

0

2 4

6 8 10 12 t (ms)

Power (kW)

Current (mA)

Problem 6. Determine the mean value of current over one complete cycle of the periodic waveforms shown in Fig. 25.9.

30 20 10

Fig. 25.9

7.0 21.5 42.0 49.5 37.0 10.0 1 2 3 4 5 6 Time (hours)

0

(a) One cycle of the trapezoidal waveform (a) is completed in 10 ms (i.e. the periodic time is 10 ms). Area under curve = area of trapezium 1 = (sum of parallel sides)(perpendicular distance 2 between parallel sides) =

195

1 {(4 + 8) × 10−3 }(5 × 10−3 ) = 30 × 10−6 As 2

Mean value over one cycle =

area under curve length of base

30 × 10−6 As = 3 mA = 10 × 10−3 s (b) One cycle of the sawtooth waveform (b) is completed in 5 ms. Area under curve = 12 (3 × 10−3 )(2) = 3 × 10−3 As Mean value over one cycle = =

area under curve length of base 3 × 10−3 As = 0.6 A 5 × 10−3 s

Fig. 25.10 (a) The time base is divided into 6 equal intervals, each of width 1 hour. Mid-ordinates are erected (shown by broken lines in Fig. 25.10) and measured.The values are shown in Fig. 25.10. Area under curve = (width of interval) (sum of mid-ordinates) = (1)[7.0 + 21.5 + 42.0 + 49.5 + 37.0 + 10.0] = 167 kWh (i.e. a measure of electrical energy) (b) Average value of waveform =

167 kWh area under curve = = 27.83 kW length of base 6h

Alternatively, average value =

Sum of mid-ordinates number of mid-ordinate

196

Basic Engineering Mathematics

Voltage (V)

Problem 8. Figure 25.11 shows a sinusoidal output voltage of a full-wave rectifier. Determine, using the mid-ordinate rule with 6 intervals, the mean output voltage.

3.6

4.0

3.5

2.9

2.2

1.7

1.6

12.0 cm

10

Fig. 25.12

0 30° 60°90° π 2

180° π

270° 3π 2

360° 2π

␪

(a) The width of each interval is

1 (2.0)[(3.6 + 1.6) + 4(4.0 + 2.9 + 1.7) 3 +2(3.5 + 2.2)] 2 = [5.2 + 34.4 + 11.4] = 34 cm2 3

area =

Fig. 25.11

One cycle of the output voltage is completed in π radians or 180◦ . The base is divided into 6 intervals, each of width 30◦ . The mid-ordinate of each interval will lie at 15◦ , 45◦ , 75◦ , etc.

(b) Mean height of ordinates =

The results are tabulated below:

34 area of diagram = length of base 12

= 2.83 cm

At 15◦ the height of the mid-ordinate is 10 sin 15◦ = 2.588 V. At 45◦ the height of the mid-ordinate is 10 sin 45◦ = 7.071 V, and so on.

12.0 cm. Using Simpson’s rule, 6

Since 1 cm represents 100 kPa, the mean pressure in the cylinder = 2.83 cm × 100 kPa/cm = 283 kPa

Height of mid-ordinate

15◦

10 sin 15◦ = 2.588 V

45◦

10 sin 45◦ = 7.071 V

◦

10 sin 75 = 9.659 V

105◦

10 sin 105◦ = 9.659 V

135◦

10 sin 135◦ = 7.071 V

165◦

10 sin 165◦ = 2.588 V

Mean or average value of output voltage =

38.636 Sum of mid-ordinates = = 6.439V number of mid-ordinate 6

(With a larger number of intervals a more accurate answer may be obtained.) For a sine wave the actual mean value is 0.637 × maximum value, which in this problem gives 6.37 V.

1. Determine the mean value of the periodic waveforms shown in Fig. 25.13 over a half cycle.

2 0

10

20 t (ms)

⫺2

100

0

5

10 t (ms)

⫺100 (a)

(b)

5 0

Problem 9. An indicator diagram for a steam engine is shown in Fig. 25.12. The base line has been divided into 6 equally spaced intervals and the lengths of the 7 ordinates measured with the results shown in centimetres. Determine (a) the area of the indicator diagram using Simpson’s rule, and (b) the mean pressure in the cylinder given that 1 cm represents 100 kPa.

Further problems on mean or average values of waveforms (Answers on page 280)

Voltage (V)

Sum of mid-ordinates = 38.636 V

Exercise 93

Current (A)

75

Now try the following exercise

◦

Current (A)

Mid-ordinate

15

30 t (ms)

⫺5 (c)

Fig. 25.13 2. Find the average value of the periodic waveforms shown in Fig. 25.14 over one complete cycle.

Irregular areas and volumes and mean values of waveforms

3. An alternating current has the following values at equal intervals of 5 ms. 10 2.6

15 4.9

20 5.8

0

30 0

4. Determine, using an approximate method, the average value of a sine wave of maximum value 50 V for (a) a half cycle and (b) a complete cycle.

2

4

6

8 10 t (ms)

5.90, 5.52, 4.22, 3.63, 3.32, 3.24, 3.16 cm Determine the area of the diagram and the mean pressure in the cylinder if 1 cm represents 90 kPa.

5 0

Fig. 25.14

25 3.5

Plot a graph of current against time and estimate the area under the curve over the 30 ms period using the mid-ordinate rule and determine its mean value.

5. An indicator diagram of a steam engine is 12 cm long. Seven evenly spaced ordinates, including the end ordinates, are measured as follows:

10

Current (A)

Voltage (mV)

Time (ms) 0 5 Current (A) 0 0.9

197

2

4

6

8 10 t (ms)

26 Triangles and some practical applications

26.1 Sine and cosine rules To ‘solve a triangle’ means ‘to find the values of unknown sides and angles’. If a triangle is right angled, trigonometric ratios and the theorem of Pythagoras may be used for its solution, as shown in Chapter 19. However, for a non-right-angled triangle, trigonometric ratios and Pythagoras’ theorem cannot be used. Instead, two rules, called the sine rule and the cosine rule, are used.

a2 = b2 + c2 − 2bc cos A or b2 = a2 + c2 − 2ac cos B or c2 = a2 + b2 − 2ab cos C The rule may be used only when: (i) 2 sides and the included angle are initially given, or (ii) 3 sides are initially given.

Sine rule With reference to triangle ABC of Fig. 26.1, the sine rule states:

26.2 Area of any triangle The area of any triangle such as ABC of Fig. 26.1 is given by:

b c a = = sinA sin B sin C

(i)

1 × base × perpendicular height, or 2

(ii)

1 ab sin C 2

(iii)

[s(s − a)(s − b)(s − c)], where

A c

B

b a

C

Fig. 26.1

s=

or

1 ac sin B 2

or

1 bc sin A, or 2

a+b+c 2

The rule may be used only when: (i) 1 side and any 2 angles are initially given, or (ii) 2 sides and an angle (not the included angle) are initially given.

Cosine rule With reference to triangle ABC of Fig. 26.1, the cosine rule states:

26.3 Worked problems on the solution of triangles and their areas Problem 1. In a triangle XYZ, ∠X = 51◦ , ∠Y = 67◦ and YZ = 15.2 cm. Solve the triangle and find its area.

Triangles and some practical applications

X z

Applying the sine rule:

51°

22.31 17.92 = sin 78◦ 51 sin C

y

67° Y x ⫽ 15.2 cm Z

Fig. 26.2

The triangle XYZ is shown in Fig. 26.2. Since the angles in a triangle add up to 180◦ , then z = 180◦ − 51◦ − 67◦ = 62◦ . Applying the sine rule: y z 15.2 = = sin 51◦ sin 67◦ sin 62◦

sin C =

from which,

Angle A = 180◦ − 78◦ 51 − 52◦ 0 = 49◦ 9 Applying the sine rule:

15.2 sin 67◦ = 18.00 cm = XZ y= sin 51◦ z 15.2 = and transposing gives: sin 51◦ sin 62◦

Using

z=

22.31 a = sin 49◦ 9 sin 78◦ 51 a=

from which,

22.31 sin 49◦ 9 = 17.20 mm sin 78◦ 51

Hence A = 49◦ 9 , C = 52◦ 0 and BC = 17.20 mm. Area of triangle ABC

15.2 sin 62◦ = 17.27 cm = XY sin 51◦

=

1 1 ac sin B = (17.20)(17.92)sin 78◦ 51 2 2 = 151.2 mm2

Area of triangle XYZ =

17.92 sin 78◦ 51 = 0.7881 22.31

Hence C = sin−1 0.7881 = 52◦ 0 or 128◦ 0 (see Chapters 19 and 20). Since B = 78◦ 51 , C cannot be 128◦ 0 , since 128◦ 0 + 78◦ 51 is greater than 180◦ . Thus only C = 52◦ 0 is valid.

15.2 y = and transposing gives: sin 51◦ sin 67◦

Using

1 1 xy sin Z = (15.2)(18.00) sin 62◦ = 120.8 cm2 2 2

1 1 (or area = xz sin Y = (15.2)(17.27) sin 67◦ = 120.8 cm2 ) 2 2

Problem 3. Solve the triangle PQR and find its area given that QR = 36.5 mm, PR = 29.6 mm and ∠Q = 36◦ Triangle PQR is shown in Fig. 26.4.

It is always worth checking with triangle problems that the longest side is opposite the largest angle, and vice-versa. In this problem, Y is the largest angle and XZ is the longest of the three sides. Problem 2. Solve the triangle ABC given B = 78◦ 51 , AC = 22.31 mm and AB = 17.92 mm. Find also its area.

P r

q ⫽ 29.6 mm

36° Q p ⫽ 36.5 mm R

Fig. 26.4 Applying the sine rule:

Triangle ABC is shown in Fig. 26.3.

36.5 29.6 = sin 36◦ sin P

c ⫽ 17 .92 mm

A

B

Fig. 26.3

199

sin P =

from which, b ⫽ 22.31 mm

Hence P = sin−1 0.7248 = 46◦ 27 ◦

When P = 46 27 78°51′ a

C

36.5 sin 36◦ = 0.7248 29.6

then

133◦ 33

Q = 36

and

R = 180◦ − 46◦ 27 − 36◦ = 97◦ 33

When P = 133◦ 33 then

or

◦

◦

Q = 36◦

and ◦

R = 180 − 133 33 − 36◦ = 10◦ 27

200

Basic Engineering Mathematics

Thus, in this problem, there are two separate sets of results and both are feasible solutions. Such a situation is called the ambiguous case. Case 1. P = 46◦ 27 , Q = 36◦ , R = 97◦ 33 , p = 36.5 mm and q = 29.6 mm. From the sine rule: 29.6 r = sin 97◦ 33 sin 36◦ 29.6 sin 97◦ 33 r= = 49.92 mm sin 36◦

from which,

1 1 Area = pq sin R = (36.5)(29.6)sin 97◦ 33 = 535.5 mm2 . 2 2

5. j = 3.85 cm, k = 3.23 cm, K = 36◦ 6. k = 46 mm, l = 36 mm, L = 35◦

26.4 Further worked problems on the solution of triangles and their areas Problem 4. Solve triangle DEF and find its area given that EF = 35.0 mm, DE = 25.0 mm and ∠E = 64◦

Triangle DEF is shown in Fig. 26.6.

Case 2. P = 133◦ 33 , Q = 36◦ , R = 10◦ 27 , p = 36.5 mm and q = 29.6 mm.

D f ⫽ 25.0 mm

From the sine rule:

64° E d ⫽ 35.0 mm F

29.6 r = sin 10◦ 27 sin 36◦ 29.6 sin 10◦ 27 = 9.134 mm sin 36◦

r=

from which,

1 1 Area = pq sin R = (36.5)(29.6)sin10◦ 27 = 97.98 mm2 . 2 2 Triangle PQR for case 2 is shown in Fig. 26.5. 133°33′ 9.134 mm Q

P

29.6 mm R 36.5 mm 36° 10°27′

Fig. 26.6 Applying the cosine rule: e2 = d 2 + f 2 − 2df cos E i.e. e2 = (35.0)2 + (25.0)2 − [2(35.0)(25.0) cos 64◦ ] = 1225 + 625 − 767.1 = 1083 √ from which, e = 1083 = 32.91 mm Applying the sine rule: 25.0 32.91 = sin 64◦ sin F

Fig. 26.5 from which, Now try the following exercise Thus Exercise 94 Further problems on the solution of triangles and their areas (Answers on page 280) In Problems 1 and 2, use the sine rule to solve the triangles ABC and find their areas. 1. A = 29◦ , B = 68◦ , b = 27 mm. ◦

◦

2. B = 71 26 , C = 56 32 , b = 8.60 cm. In Problems 3 and 4, use the sine rule to solve the triangles DEF and find their areas.

e

sin F =

25.0 sin 64◦ = 0.6828 32.91

∠F = sin−1 0.6828 = 43◦ 4

or

136◦ 56

F = 136◦ 56 is not possible in this case since 136◦ 56 + 64◦ is greater than 180◦ . Thus only F = 43◦ 4 is valid. ∠D = 180◦ − 64◦ − 43◦ 4 = 72◦ 56 Area of triangle DEF = 12 df sin E = 12 (35.0)(25.0) sin 64◦ = 393.2 mm2 Problem 5. A triangle ABC has sides a = 9.0 cm, b = 7.5 cm and c = 6.5 cm. Determine its three angles and its area.

3. d = 17 cm, f = 22 cm, F = 26◦ 4. d = 32.6 mm, e = 25.4 mm, D = 104◦ 22 In Problems 5 and 6, use the sine rule to solve the triangles JKL and find their areas.

Triangle ABC is shown in Fig. 26.7. It is usual first to calculate the largest angle to determine whether the triangle is acute or obtuse. In this case the largest angle is A (i.e. opposite the longest side).

Triangles and some practical applications

A

Applying the cosine rule:

c ⫽ 6.5 cm

b ⫽ 7.5 cm a ⫽ 9.0 cm

B

= 4.52 + 7.22 − [2(4.5)(7.2) cos 128◦ ] = 20.25 + 51.84 − [−39.89]

Applying the cosine rule: a2 = b2 + c2 − 2bc cos A

=

7.2 10.58 = ◦ sin 128 sin Z

b2 + c2 − a2 2bc 7.52 + 6.52 − 9.02 = 0.1795 2(7.5)(6.5)

Hence A = cos−1 0.1795 = 79◦ 40 (or 280◦ 20 , which is obviously impossible). The triangle is thus acute angled since cos A is positive. (If cos A had been negative, angle A would be obtuse, i.e. lie between 90◦ and 180◦ ). Applying the sine rule: 7.5 9.0 = sin 79◦ 40 sin B from which,

= 20.25 + 51.84 + 39.89 = 112.0 √ y = 112.0 = 10.58 cm Applying the sine rule:

from which, 2bc cos A = b2 + c2 − a2 cos A =

y2 = x2 + z 2 − 2xz cos Y

C

Fig. 26.7

and

sin B =

201

from which,

sin Z =

7.2 sin 128◦ = 0.5363 10.58

Hence Z = sin−1 0.5363 = 32◦ 26 (or 147◦ 34 which, here, is impossible). X = 180◦ − 128◦ − 32◦ 26 = 19◦ 34 1 1 Area = xz sin Y = (4.5)(7.2) sin 128◦ = 12.77 cm2 2 2

Now try the following exercise ◦

7.5 sin 79 40 = 0.8198 9.0

Hence B = sin−1 0.8198 = 55◦ 4 C = 180◦ − 79◦ 40 − 55◦ 4 = 45◦ 16 Area = [s(s − a)(s − b)(s − c)], where and

9.0 + 7.5 + 6.5 a+b+c = = 11.5 cm s= 2 2 Hence area = [11.5(11.5 − 9.0)(11.5 − 7.5)(11.5 − 6.5)] = [11.5(2.5)(4.0)(5.0)] = 23.98 cm2 Alternatively, area = 12 ab sin C = 12 (9.0)(7.5) sin 45◦ 16

Exercise 95

Further problems on the solution of triangles and their areas (Answers on page 280)

In Problems 1 and 2, use the cosine and sine rules to solve the triangles PQR and find their areas. 1. q = 12 cm, r = 16 cm, P = 54◦ 2. q = 3.25 m, r = 4.42 m, P = 105◦ In Problems 3 and 4, use the cosine and sine rules to solve the triangles XYZ and find their areas. 3. x = 10.0 cm, y = 8.0 cm, z = 7.0 cm. 4. x = 21 mm, y = 34 mm, z = 42 mm.

= 23.98 cm2 Problem 6. Solve triangle XYZ, shown in Fig. 26.8, and find its area given that Y = 128◦ , XY = 7.2 cm and YZ = 4.5 cm. X y z ⫽ 7.2 cm

There are a number of practical situations where the use of trigonometry is needed to find unknown sides and angles of triangles. This is demonstrated in the following worked problems.

128° Y

Fig. 26.8

26.5 Practical situations involving trigonometry

x ⫽ 4.5 cm

Z

Problem 7. A room 8.0 m wide has a span roof which slopes at 33◦ on one side and 40◦ on the other. Find the length of the roof slopes, correct to the nearest centimetre.

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Basic Engineering Mathematics

Hence C = sin−1 0.3518 = 20◦ 36 (or 159◦ 24 , which is impossible in this case). B = 180◦ − 60◦ − 20◦ 36 = 99◦ 24 .

B 33°

A

40° 8.0 m

C

Fig. 26.9 A section of the roof is shown in Fig. 26.9. Angle at ridge, B = 180◦ − 33◦ − 40◦ = 107◦ From the sine rule: a 8.0 = ◦ sin 107 sin 33◦ a=

from which,

Applying the sine rule again: b 80 = sin 60◦ sin 99◦ 24 80 sin 99◦ 24 from which, b= = 91.14 km sin 60◦ Since the cyclist travels 91.14 km in 5 hours then average speed =

8.0 sin 33◦ = 4.556 m sin 107◦

Also from the sine rule: c 8.0 = sin 107◦ sin 40◦

91.14 distance = = 18.23 km/h time 5

Problem 9. Two voltage phasors are shown in Fig. 26.11. If V1 = 40 V and V2 = 100 V determine the value of their resultant (i.e. length OA) and the angle the resultant makes with V1 A V2 ⫽ 100 V

◦

8.0 sin 40 = 5.377 m sin 107◦ Hence the roof slopes are 4.56 m and 5.38 m, correct to the nearest centimetre. c=

from which,

45° C B V1 ⫽ 40 V

Problem 8. A man leaves a point walking at 6.5 km/h in a direction E 20◦ N (i.e. a bearing of 70◦ ). A cyclist leaves the same point at the same time in a direction E 40◦ S (i.e. a bearing of 130◦ ) travelling at a constant speed. Find the average speed of the cyclist if the walker and cyclist are 80 km apart after 5 hours.

Fig. 26.11 Angle OBA = 180◦ − 45◦ = 135◦ Applying the cosine rule: OA2 = V12 + V22 − 2V1 V2 cos OBA = 402 + 1002 − {2(40)(100) cos 135◦ }

After 5 hours the walker has travelled 5 × 6.5 = 32.5 km (shown as AB in Fig. 26.10). If AC is the distance the cyclist travels in 5 hours then BC = 80 km. The resultant

20° N

m 5k B

A

E 40°

100 131.4 = sin 135◦ sin AOB

80 km

S

= 1600 + 10 000 + 5657 = 17 257 √ OA = 17 257 = 131.4 V

Applying the sine rule:

32. W

= 1600 + 10 000 − {−5657}

b

from which, C

Fig. 26.10

sin AOB =

100 sin 135◦ = 0.5381 131.4

Hence angle AOB = sin−1 0.5381 = 32◦ 33 (or 147◦ 27 , which is impossible in this case). Hence the resultant voltage is 131.4 volts at 32◦ 33 to V1 .

Applying the sine rule: 32.5 80 = sin 60◦ sin C from which,

sin C =

32.5 sin 60◦ = 0.3518 80

Problem 10. In Fig. 26.12, PR represents the inclined jib of a crane and is 10.0 m long. PQ is 4.0 m long. Determine the inclination of the jib to the vertical and the length of tie QR.

Triangles and some practical applications

R

Q

203

3. A jib crane is shown in Fig. 26.13. If the tie rod PR is 8.0 long and PQ is 4.5 m long determine (a) the length of jib RQ and (b) the angle between the jib and the tie rod.

120° 4.0 m 10.0 m R

P

Fig. 26.12 130° P

Applying the sine rule: Q

PQ PR = sin 120◦ sin R from which,

sin R =

PQ sin 120◦ PR

(4.0) sin 120◦ = 0.3464 = 10.0

Fig. 26.13 4. A building site is in the form of a quadrilateral as shown in Fig. 26.14, and its area is 1510 m2 . Determine the length of the perimeter of the site.

Hence ∠R = sin−1 0.3464 = 20◦ 16 (or 159◦ 44 , which is impossible in this case).

28.5 m 72°

∠P = 180◦ − 120◦ − 20◦ 16 = 39◦ 44 , which is the inclination of the jib to the vertical.

34.6 m

Applying the sine rule:

75°

52.4 m

10.0 QR = sin 120◦ sin 39◦ 44 from which, length of tie, QR =

10.0 sin 39◦ 44 sin 120◦

= 7.38 m

Fig. 26.14 5. Determine the length of members BF and EB in the roof truss shown in Fig. 26.15.

Now try the following exercise 4m

Exercise 96 Further problems on practical situations involving trigonometry (Answers on page 281) 1. A ship P sails at a steady speed of 45 km/h in a direction of W 32◦ N (i.e. a bearing of 302◦ ) from a port. At the same time another ship Q leaves the port at a steady speed of 35 km/h in a direction N 15◦ E (i.e. a bearing of 015◦ ). Determine their distance apart after 4 hours. 2. Two sides of a triangular plot of land are 52.0 m and 34.0 m, respectively. If the area of the plot is 620 m2 find (a) the length of fencing required to enclose the plot and (b) the angles of the triangular plot.

2.5 m

E

F

50° A 5m

4m D

B

2.5 m 50° 5m C

Fig. 26.15 6. A laboratory 9.0 m wide has a span roof which slopes at 36◦ on one side and 44◦ on the other. Determine the lengths of the roof slopes. 7. PQ and QR are the phasors representing the alternating currents in two branches of a circuit. Phasor PQ is 20.0 A and is horizontal. Phasor QR (which is joined to the end of PQ to form triangle PQR) is 14.0 A and is at an angle of 35◦ to the horizontal. Determine the resultant phasor PR and the angle it makes with phasor PQ.

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Basic Engineering Mathematics

26.6 Further practical situations involving trigonometry

A

m

30.0 c

10.0 cm

50°

O

B

Problem 11. vertical aerial stands on horizontal ground. A surveyor positioned due east of the aerial measures the elevation of the top as 48◦ . He moves due south 30.0 m and measures the elevation as 44◦ . Determine the height of the aerial.

In Fig. 26.16, DC represents the aerial, A is the initial position of the surveyor and B is final position. From triangle ACD, DC DC tan 48◦ = , from which AC = AC tan 48◦

Fig. 26.17 (a) For the position shown in Fig. 26.17 determine the angle between the connecting rod AB and the horizontal and the length of OB. (b) How far does B move when angle AOB changes from 50◦ to 120◦ ? (a) Applying the sine rule: AB AQ = sin 50◦ sin B

D

sin B =

from which,

AO sin 50◦ 10.0 sin 50◦ = AB 30.0

= 0.2553 48°

C

−1

A 30.0 m

44° B

Hence B = sin 0.2553 = 14◦ 47 (or 165◦ 13 , which is impossible in this case). Hence the connecting rod AB makes an angle of 14◦ 47 with the horizontal. Angle OAB = 180◦ − 50◦ − 14◦ 47 = 115◦ 13 Applying the sine rule:

Fig. 26.16 DC tan 44◦ For triangle ABC, using Pythagoras’ theorem:

30.0 OB = sin 50◦ sin 115◦ 13

Similarly, from triangle BCD, BC =

BC 2 = AB2 + AC 2 2 2 DC DC 2 = (30.0) + tan 44◦ tan 48◦ 1 1 DC 2 − = 30.02 tan2 44◦ tan2 48◦

from which,

OB =

(b) Figure 26.18 shows the initial and final positions of the crank mechanism. In triangle OA B , applying the sine rule: 30.0 10.0 = ◦ sin A B O sin 120 from which, sin A B O =

DC 2 (1.072323 − 0.810727) = 30.02 30.0 cm

DC 2 =

30.0 sin 115◦ 13 = 35.43 cm sin 50◦

10.0 sin 120◦ = 0.2887 30.0 A

2

30.0 = 3440.4 0.261596

50° B

√ Hence, height of aerial, DC = 3340.4 = 58.65 m.

B′

A′ 120° 10.0 cm O

Fig. 26.18 Problem 12. A crank mechanism of a petrol engine is shown in Fig. 26.17. Arm OA is 10.0 cm long and rotates clockwise about 0. The connecting rod AB is 30.0 cm long and end B is constrained to move horizontally.

Hence A B O = sin−1 0.2887 = 16◦ 47 (or 163◦ 13 which is impossible in this case). Angle OA B = 180◦ − 120◦ − 16◦ 47 = 43◦ 13

Triangles and some practical applications

B

Applying the sine rule:

from which,

OB 30.0 = sin 120◦ sin 43◦ 13 30.0 sin 43◦ 13 OB = = 23.72 cm sin 120◦

Since OB = 35.43 cm and OB = 23.72 cm then BB = 35.43 − 23.72 = 11.71 cm Hence B moves 11.71 cm when angle AOB changes from 50◦ to 120◦ . Problem 13. The area of a field is in the form of a quadrilateral ABCD as shown in Fig. 26.19. Determine its area. B

205

42.5 m 56°

C

39.8 m 62.3 m A

114° 21.4 m

A

40° C

Fig. 26.20 determine the length of AC and the angle between the crank and the connecting rod. 4. From Fig. 26.20, determine how far C moves, correct to the nearest millimetre when angle CAB changes from 40◦ to 160◦ , B moving in an anticlockwise direction. 5. A surveyor, standing W 25◦ S of a tower measures the angle of elevation of the top of the tower as 46◦ 30 . From a position E 23◦ S from the tower the elevation of the top is 37◦ 15 . Determine the height of the tower if the distance between the two observations is 75 m. 6. Calculate, correct to 3 significant figures, the co-ordinates x and y to locate the hole centre at P shown in Fig. 26.21. P

D

Fig. 26.19 y

A diagonal drawn from B to D divides the quadrilateral into two triangles. Area of quadrilateral ABCD = area of triangle ABD + area of triangle BCD =

1 (39.8)(21.4) sin 114◦ 2

140°

116°

+

1 (42.5)(62.3) sin 56◦ 2

= 389.04 + 1097.5 = 1487 m

2

Now try the following exercise

x

Fig. 26.21 7. An idler gear, 30 mm in diameter, has to be fitted between a 70 mm diameter driving gear and a 90 mm diameter driven gear as shown in Fig. 26.22. Determine the value of angle θ between the centre lines. 90 mm dia

Exercise 97 Further problems on practical situations involving trigonometry (Answers on page 281) 1. Three forces acting on a fixed point are represented by the sides of a triangle of dimensions 7.2 cm, 9.6 cm and 11.0 cm. Determine the angles between the lines of action and the three forces. 2. A vertical aerial AB, 9.60 m high, stands on ground which is inclined 12◦ to the horizontal. A stay connects the top of the aerial A to a point C on the ground 10.0 m downhill from B, the foot of the aerial. Determine (a) the length of the stay, and (b) the angle the stay makes with the ground. 3. A reciprocating engine mechanism is shown in Fig. 26.20. The crank AB is 12.0 cm long and the connecting rod BC is 32.0 cm long. For the position shown

100 mm

99.78 mm u

30 mm dia 70 mm dia

Fig. 26.22 8. 16 holes are equally spaced opn a pitch circle of 70 mm diameter. Determine the length of the chord joining the centres of two adjacent holes.

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Basic Engineering Mathematics

Assignment 12

4. A triangular plot of land ABC is shown in Fig. A12.1. Solve the triangle and determine its area. (10)

This assignment covers the material in Chapters 25 and 26. The marks for each question are shown in brackets at the end of each question. 1. Plot a graph of y = 3x2 + 5 from x = 1 to x = 4. Estimate, correct to 2 decimal places, using 6 intervals, the area enclosed by the curve, the ordinates x = 1 and x = 4, and the x-axis by (a) the trapezoidal rule, (b) the mid-ordinate rule, and (c) Simpson’s rule. (12) 2. A circular cooling tower is 20 m high. The inside diameter of the tower at different heights is given in the following table: Height (m) Diameter (m)

0 16.0

5.0 13.3

10.0 10.7

15.0 8.6

20.0 8.0

Determine the area corresponding to each diameter and hence estimate the capacity of the tower in cubic metres. (7)

A 15 m

2 2.4

3 3.7

4 5.2

5 6.0

15.4 m

B

C

Fig. A12.1 5. A car is travelling 20 m above sea level. It then travels 500 m up a steady slope of 17◦ . Determine, correct to the nearest metre, how high the car is now above see level. (3) 6. Figure A12.2 shows a roof truss PQR with rafter PQ = 3 m. Calculate the length of (a) the roof rise PP , (b) rafter PR, and (c) the roof span QR. Find also (d) the cross-sectional area of the roof truss. (12)

3. A vehicle starts from rest and its velocity is measured every second for 6 seconds, with the following results: Time t (s) 0 1 Velocity v (m/s) 0 1.2

P 3m

6 9.2

40°

Using Simpson’s rule, calculate (a) the distance travelled in 6 s (i.e. the area under the v/t graph) and (b) the average speed over this period. (6)

71°

Q

Fig. A12.2

32° P′

R

27 Vectors

27.1 Introduction Some physical quantities are entirely defined by a numerical value and are called scalar quantities or scalars. Examples of scalars include time, mass, temperature, energy and volume. Other physical quantities are defined by both a numerical value and a direction in space and these are called vector quantities or vectors. Examples of vectors include force, velocity, moment and displacement.

(ii) two capital letters with an arrow above them to denote the − → sense of direction, e.g. AB, where A is the starting point and B the end point of the vector, (iii) a line over the top of letters, e.g. AB or a, ¯ (iv) letters with an arrow above, e.g. a, A, (v) underlined letters, e.g. a, (vi) xi + jy, where i and j are axes at right-angles to each other; for example, 3i + 4j means 3 units in the i direction and 4 units in the j direction, as shown in Fig. 27.2.

27.2 Vector addition j

A vector may be represented by a straight line, the length of line being directly proportional to the magnitude of the quantity and the direction of the line being in the same direction as the line of action of the quantity. An arrow is used to denote the sense of the vector, that is, for a horizontal vector, say, whether it acts from left to right or vice-versa. The arrow is positioned at the end of the vector and this position is called the ‘nose’ of the vector. Figure 27.1 shows a velocity of 20 m/s at an angle of 45◦ to the horizontal and may be depicted by oa = 20 m/s at 45◦ to the horizontal.

4

3

2 1

O

a 20 m/s

o

45°

Fig. 27.1 To distinguish between vector and scalar quantities, various ways are used. These include: (i) bold print,

A(3, 4)

1

2

3

i

Fig. 27.2 a ; for example, the vector OA shown b 3 in Fig. 27.2 could be represented by 4

(vii) a column matrix

− → Thus, in Fig. 27.2, OA ≡ OA ≡ OA ≡ 3i + 4j ≡

3 4

208

Basic Engineering Mathematics

The one adopted in this text is to denote vector quantities in bold print. Thus, oa represents a vector quantity, but oa is the magnitude of the vector oa. Also, positive angles are measured in an anticlockwise direction from a horizontal, right facing line and negative angles in a clockwise direction from this line – as with graphical work. Thus 90◦ is a line vertically upwards and −90◦ is a line vertically downwards. The resultant of adding two vectors together, say V1 at an angle θ1 and V2 at angle (−θ2 ), as shown in Fig. 27.3(a), can be obtained by drawing oa to represent V1 and then drawing ar to represent V2 . The resultant of V1 + V2 is given by or. This is shown in Fig. 27.3(b), the vector equation being oa + ar = or. This is called the ‘nose-to-tail’ method of vector addition.

Scale in Newtons 0

2

4N O

4

6

45° 7N (a) R

r 4N 45°

o

O

a

7N

4N

45° 7N

(b) a V1

V1

u1 u2

r

o

V2

O

(c)

Fig. 27.4 u1

R u2 V2

(a)

(b)

(c)

Fig. 27.3

Problem 2. Use a graphical method to determine the magnitude and direction of the resultant of the three velocities shown in Fig. 27.5.

Alternatively, by drawing lines parallel to V1 and V2 from the noses of V2 and V1 , respectively, and letting the point of intersection of these parallel lines be R, gives OR as the magnitude and direction of the resultant of adding V1 and V2 , as shown in Fig. 27.3(c). This is called the ‘parallelogram’ method of vector addition.

v2

15 m/s

Problem 1. A force of 4 N is inclined at an angle of 45◦ to a second force of 7 N, both forces acting at a point. Find the magnitude of the resultant of these two forces and the direction of the resultant with respect to the 7 N force by both the ‘triangle’ and the ‘parallelogram’ methods.

10 m/s 20° 10° v3

v1

7 m/s

Fig. 27.5

The forces are shown in Fig. 27.4(a). Although the 7 N force is shown as a horizontal line, it could have been drawn in any direction. Using the ‘nose-to-tail’ method, a line 7 units long is drawn horizontally to give vector oa in Fig. 27.4(b). To the nose of this vector ar is drawn 4 units long at an angle of 45◦ to oa. The resultant of vector addition is or and by measurement is 10.2 units long and at an angle of 16◦ to the 7 N force. Figure 27.4(c) uses the ‘parallelogram’ method in which lines are drawn parallel to the 7 N and 4 N forces from the noses of the 4 N and 7 N forces, respectively. These intersect at R. Vector OR gives the magnitude and direction of the resultant of vector addition and as obtained by the ‘nose-to-tail’ method is 10.2 units long at an angle of 16◦ to the 7 N force.

Often it is easier to use the ‘nose-to-tail’ method when more than two vectors are being added. The order in which the vectors are added is immaterial. In this case the order taken is v1 , then v2 , then v3 but just the same result would have been obtained if the order had been, say, v1 , v3 and finally v2 . v1 is drawn 10 units long at an angle of 20◦ to the horizontal, shown by oa in Fig. 27.6. v2 is added to v1 by drawing a line 15 units long vertically upwards from a, shown as ab. Finally, v3 is added to v1 + v2 by drawing a line 7 units long at an angle at 190◦ from b, shown as br. The resultant of vector addition is or and by measurement is 17.5 units long at an angle of 82◦ to the horizontal. Thus v 1 + v 2 + v 3 = 17.5 m/s at 82◦ to the horizontal.

Vectors

10° b r

209

Having obtained √ H and V , the magnitude of the resultant vector R is given by H 2 + V 2 and its angle to the horizontal is given by tan−1 (V /H ) Problem 3. Resolve the acceleration vector of 17 m/s2 at an angle of 120◦ to the horizontal into a horizontal and a vertical component.

0 2 4 6 8 10 12 Scale in m/s a

For a vector A at angle θ to the horizontal, the horizontal component is given by A cos θ and the vertical component by A sin θ. Any convention of signs may be adopted, in this case horizontally from left to right is taken as positive and vertically upwards is taken as positive.

82° 20°

o

Fig. 27.6

Horizontal component H = 17 cos 120◦ = −8.50 m/s2 , acting from left to right.

27.3 Resolution of vectors A vector can be resolved into two component parts such that the vector addition of the component parts is equal to the original vector. The two components usually taken are a horizontal component and a vertical component. For the vector shown as F in Fig. 27.7, the horizontal component is F cos θ and the vertical component is F sin θ.

Vertical component V = 17 sin 120◦ = 14.72 m/s2 , acting vertically upwards. These component vectors are shown in Fig. 27.9. ⫹V 17 m/s2 14.72 m/s2

F

F sin u u

120°

F cos u

⫺H

Fig. 27.7

+H

8.50 m/s2

For the vectors F1 and F2 shown in Fig. 27.8, the horizontal component of vector addition is: H = F1 cos θ1 + F2 cos θ2 and the vertical component of vector addition is:

−V

V = F1 sin θ1 + F2 sin θ2

Fig. 27.9

V

F1 sin u1 F2 sin u2

Problem 4. Calculate the resultant force of the two forces given in Problem 1. F1 u1

F2

With reference to Fig. 27.4(a): u2

F1 cos u1 F2 cos u2

Fig. 27.8

Horizontal component of force,

H = 7 cos 0◦ + 4 cos 45◦ = 7 + 2.828 = 9.828 N

H

Vertical component of force,

V = 7 sin 0◦ + 4 sin 45◦ = 0 + 2.828 = 2.828 N

210

Basic Engineering Mathematics

The magnitude of the resultant of vector addition = H 2 + V 2 = 9.8282 + 2.8282 √ = 104.59 = 10.23 N

4. Three forces of 2 N, 3 N and 4 N act as shown in Fig. 27.10. Calculate the magnitude of the resultant force and its direction relative to the 2 N force. 4N

The direction of the resultant of vector addition V 2.828 = tan−1 = 16.05◦ = tan−1 H 9.828 Thus, the resultant of the two forces is a single vector of 10.23 N at 16.05◦ to the 7 N vector. 60°

Problem 5. Calculate the resultant velocity of the three velocities given in Problem 2.

60°

With reference to Fig. 27.5: Horizontal component of the velocity, H = 10 cos 20◦ + 15 cos 90◦ + 7 cos 190◦ = 9.397 + 0 + (−6.894) = 2.503 m/s Vertical component of the velocity, V = 10 sin 20◦ + 15 sin 90◦ + 7 sin 190◦ = 3.420 + 15 + (−1.216) = 17.204 m/s Magnitude of the resultant of vector addition = H 2 + V 2 = 2.5032 + 17.2042 √ = 302.24 = 17.39 m/s Direction of the resultant of vector addition V 17.204 = tan−1 = tan−1 H 2.503 = tan−1 6.8734 = 81.72◦ Thus, the resultant of the three velocities is a single vector of 17.39 m/s at 81.72◦ to the horizontal

3N

2N

Fig. 27.10

5. A load of 5.89 N is lifted by two strings, making angles of 20◦ and 35◦ with the vertical. Calculate the tensions in the strings. [For a system such as this, the vectors representing the forces form a closed triangle when the system is in equilibrium]. 6. The acceleration of a body is due to four component, coplanar accelerations. These are 2 m/s2 due north, 3 m/s2 due east, 4 m/s2 to the south-west and 5 m/s2 to the south-east. Calculate the resultant acceleration and its direction. 7. A current phasor i1 is 5 A and horizontal.A second phasor i2 is 8 A and is at 50◦ to the horizontal. Determine the resultant of the two phasors, i1 + i2 , and the angle the resultant makes with current i1 . 8. An object is acted upon by two forces of magnitude 10 N and 8 N at an angle of 60◦ to each other. Determine the resultant force on the object. 9. A ship heads on a course due south at 12 km/h. It is moved off course by a current flowing in a north-easterly direction at 8 km/h. Determine the resultant velocity.

Now try the following exercise Exercise 98 Further problems on vectors (Answers on page 281) 1. Forces of 23 N and 41 N act at a point and are inclined at 90◦ to each other. Find, by drawing, the resultant force and its direction relative to the 41 N force. 2. Forces A, B and C are coplanar and act at a point. Force A is 12 kN at 90◦ , B is 5 kN at 180◦ and C is 13 kN at 293◦ . Determine graphically the resultant force. 3. Calculate the magnitude and direction of velocities of 3 m/s at 18◦ and 7 m/s at 115◦ when acting simultaneously on a point.

27.4 Vector subtraction In Fig. 27.11, a force vector F is represented by oa. The vector (−oa) can be obtained by drawing a vector from o in the opposite sense to oa but having the same magnitude, shown as ob in Fig. 27.11, i.e. ob = (−oa). For two vectors acting at a point, as shown in Fig. 27.12(a), the resultant of vector addition is os = oa + ob. Fig. 27.12(b) shows vectors ob + (−oa), that is, ob − oa and the vector equation is ob − oa = od. Comparing od in Fig. 27.12(b) with the broken line ab in Fig. 27.12(a) shows that the second diagonal of the ‘parallelogram’ method of vector addition gives the magnitude and direction of vector subtraction of oa from ob.

Vectors

a

F

Magnitude of a 1 + a2 =

Fig. 27.11 d

s

o

b

⫺a

a

a

o

(a)

(−2.13)2 + 2.992 = 3.67 m/s2 2.99 −1 Direction of a1 + a2 = tan and must lie in the −2.13 second quadrant since H is negative and V is positive. 2.99 tan−1 = −54.53◦ , and for this to be in the second −2.13 quadrant, the true angle is 180◦ displaced, i.e. 180◦ − 54.53◦ or 125.47◦

b

b

Vertical component of a1 + a2 , V = 1.5 sin 90◦ + 2.6 sin 145◦ = 2.99

o

⫺F

211

Thus a1 + a2 = 3.67 m/s2 at 125.47◦ .

(b)

Horizontal component of a1 − a2 , that is, a1 + (−a2 )

Fig. 27.12

= 1.5 cos 90◦ + 2.6 cos(145◦ − 180◦ ) Problem 6. Accelerations of a1 = 1.5 m/s2 at 90◦ and a2 = 2.6 m/s2 at 145◦ act at a point. Find a1 + a2 and a1 − a2 by (i) drawing a scale vector diagram and (ii) by calculation.

(i) The scale vector diagram is shown in Fig. 27.13. By measurement, a1 + a2 = 3.7 m/s2 at 126◦ a1 − a2 = 2.1 m/s at 0 2

Vertical component of a1 − a2 , that is, a1 + (−a2 ) = 1.5 sin 90◦ + 2.6 sin(−35◦ ) = 0 √ Magnitude of a1 − a2 = 2.132 + 02 = 2.13 m/s2 0 Direction of a1 − a2 = tan−1 = 0◦ 2.13 Thus a1 − a2 = 2.13 m/s2 at 0◦

◦

a1 ⫹ a 2 0

= 2.6 cos(−35◦ ) = 2.13

1

2

Scale in m/s2

3

Problem 7. Calculate the resultant of (i) v 1 − v 2 + v 3 and (ii) v 2 − v 1 − v 3 when v 1 = 22 units at 140◦ , v 2 = 40 units at 190◦ and v 3 = 15 units at 290◦ (i) The vectors are shown in Fig. 27.14.

a1 a2

⫹V 1.5 2.6 m/s2

m/s2

145°

126°

a1 ⫺ a2

22 190° ⫺H

140° ⫹H

290° 40

⫺a2

15

Fig. 27.13 (ii) Resolving horizontally and vertically gives: Horizontal component of a1 + a2 , H = 1.5 cos 90◦ + 2.6 cos 145◦ = −2.13

⫺V

Fig. 27.14

212

Basic Engineering Mathematics

The horizontal component of v 1 − v 2 + v 3 = (22 cos 140◦ ) − (40 cos 190◦ ) + (15 cos 290◦ ) = (−16.85) − (−39.39) + (5.13) = 27.67 units The vertical component of v 1 − v 2 + v 3 = (22 sin 140◦ ) − (40 sin 190◦ ) + (15 sin 290◦ ) = (14.14) − (−6.95) + (−14.10) = 6.99 units The magnitude of the resultant, R, which can be represented by the mathematical symbol for ‘the modulus of’ as |v1 − v2 + v3 | is given by: |R| = 27.672 + 6.992 = 28.54 units The direction of the resultant, R, which can be represented by the mathematical symbol for ‘the argument of’ as arg (v1 − v2 + v3 ) is given by: 6.99 = 14.18◦ arg R = tan−1 27.67 Thus v 1 − v 2 + v 3 = 28.54 units at 14.18◦

27.5 Relative velocity For relative velocity problems, some fixed datum point needs to be selected. This is often a fixed point on the earth’s surface. In any vector equation, only the start and finish points affect the resultant vector of a system. Two different systems are shown in Fig. 27.15, but in each of the systems, the resultant vector is ad. The vector equation of the system shown in Fig. 27.15(a) is: ad = ab + bd and that for the system shown in Fig. 27.15(b) is: ad = ab + bc + cd b b c

(a)

(ii) The horizontal component of v 2 − v 1 − v 3 = (40 cos 190◦ ) − (22 cos 140◦ ) − (15 cos 290◦ ) = (−39.39) − (−16.85) − (5.13) = −27.67 units The vertical component of v 2 − v 1 − v 3 = (40 sin 190◦ ) − (22 sin 140◦ ) − (15 sin 290◦ ) = (−6.95) − (14.14) − (−14.10) = −6.99 units 2 + (−6.99)2 = Let R = v2 − v1 − v3 then |R|= (−27.67) −6.99 and must lie in the 28.54 units and arg R = tan−1 −27.67 third quadrant since both H and V are negative quantities. −6.99 = 14.18◦ , hence the required angle is Tan−1 −27.67 180◦ + 14.18◦ = 194.18◦

d

a

a

d (b)

Fig. 27.15 Thus in vector equations of this form, only the first and last letters, a and d, respectively, fix the magnitude and direction of the resultant vector. This principle is used in relative velocity problems.

Problem 8. Two cars, P and Q, are travelling towards the junction of two roads which are at right angles to one another. Car P has a velocity of 45 km/h due east and car Q a velocity of 55 km/h due south. Calculate (i) the velocity of car P relative to car Q, and (ii) the velocity of car Q relative to car P.

Thus v 2 − v 1 − v 3 = 28.54 units at 194.18◦ This result is as expected, since v 2 − v 1 − v 3 = −(v 1 − v 2 + v 3 ) and the vector 28.54 units at 194.18◦ is minus times the vector 28.54 units at 14.18◦

Now try the following exercise Exercise 99 Further problems on vectors (Answers on page 281) 1. Forces of F1 = 40 N at 45◦ and F2 = 30 N at 125◦ act at a point. Determine by drawing and by calculation (a) F1 + F2 (b) F1 − F2 2. Calculate the resultant of (a) v 1 + v 2 − v 3 (b) v 3 − v 2 + v 1 when v 1 = 15 m/s at 85◦ , v 2 = 25 m/s at 175◦ and v 3 = 12 m/s at 235◦

(i) The directions of the cars are shown in Fig. 27.16 (a), called a space diagram. The velocity diagram is shown in Fig. 27.16 (b), in which pe is taken as the velocity of car P relative to point e on the earth’s surface. The velocity of P relative to Q is vector pq and the vector equation is pq = pe + eq. Hence the vector directions are as shown, eq being in the opposite direction to qe. From the geometry of the vector triangle, |pq| = 452 + 552 = 71.06 km/h and 55 arg pq = tan−1 = 50.71◦ 45 i.e., the velocity of car P relative to car Q is 71.06 km/h at 50.71◦ (ii) The velocity of car Q relative to car P is given by the vector equation qp = qe + ep and the vector diagram is as shown

Vectors

N W

q

q

E

213

Thus the velocity of car Q relative to car P is 71.06 km/h at 230.71◦

S P

Q

55 km/h

Now try the following exercise p

e

45 km/h (a)

p

e

Exercise 100 (b)

(c)

Fig. 27.16 in Fig. 27.16(c), having ep opposite in direction to pe. From the geometry of this vector triangle: | qp | = 452 + 552 = 71.06 km/h and 55 arg qp = tan−1 = 50.71◦ 45 but must lie in the third quadrant, i.e., the required angle is 180◦ + 50.71◦ = 230.71◦

Further problems on relative velocity (Answers on page 281)

1. A car is moving along a straight horizontal road at 79.2 km/h and rain is falling vertically downwards at 26.4 km/h. Find the velocity of the rain relative to the driver of the car. 2. Calculate the time needed to swim across a river 142 m wide when the swimmer can swim at 2 km/h in still water and the river is flowing at 1 km/h. 3. Object P is moving at 10 m/s in a westerly direction. Object Q is moving at 15 m/s in a south-westerly direction. Determine the velocity of Q relative to P.

28 Adding of waveforms

28.1 Combination of two periodic functions There are a number of instances in engineering and science where waveforms combine and where it is required to determine the single phasor (called the resultant) that could replace two or more separate phasors. (A phasor is a rotating vector). Uses are found in electrical alternating current theory, in mechanical vibrations, in the addition of forces and with sound waves. There are several methods of determining the resultant and two such methods – plotting/measuring, and resolution of phasors by calculation – are explained in this chapter. Knowledge of chapters 19, 20 and 26 involving trigonometry and waveforms is needed for this chapter.

34° y 3.6 3

y1 ⫽ 3 sin A yR ⫽ 3.6 sin(A ⫹ 34°)

2

y2 ⫽ 2 cos A

1 0

90°

180°

270°

360°

A

⫺1 ⫺2 ⫺3

28.2 Plotting periodic functions This may be achieved by sketching the separate functions on the same axes and then adding (or subtracting) ordinates at regular intervals. This is demonstrated in worked problems 1 to 3. Problem 1. Plot the graph of y1 = 3 sin A from A = 0◦ to A = 360◦ . On the same axes plot y2 = 2 cos A. By adding ordinates plot yR = 3 sin A + 2 cos A and obtain a sinusoidal expression for this resultant waveform y1 = 3 sin A and y2 = 2 cos A are shown plotted in Fig. 28.1. Ordinates may be added at, say, 15◦ intervals. For example, at 0◦ , ◦

at 15 ,

y1 + y2 = 0 + 2 = 2 y1 + y2 = 0.78 + 1.93 = 2.71

at 120◦ , y1 + y2 = 2.60 + (−1) = 1.6 at 210◦ , y1 + y2 = −1.50 − 1.73 = −3.23, and so on

Fig. 28.1 The resultant waveform, shown by the broken line, has the same period, i.e. 360◦ , and thus the same frequency as the single phasors. The maximum value, or amplitude, of the resultant is 3.6. The resultant waveform leads y1 = 3 sin A by 34◦ or 0.593 rad. The sinusoidal expression for the resultant waveform is: yR = 3.6 sin(A + 34◦ )

or yR = 3.6 sin( A + 0.593)

Problem 2. Plot the graphs of y1 = 4 sin ωt and y2 = 3 sin (ωt − π/3) on the same axes, over one cycle. By adding ordinates at intervals plot yR = y1 + y2 and obtain a sinusoidal expression for the resultant waveform y1 = 4 sin ωt and y2 = 3 sin (ωt − π/3) are shown plotted in Fig. 28.2. Ordinates are added at 15◦ intervals and the resultant is shown by the broken line. The amplitude of the resultant is 6.1 and it lags y1 by 25◦ or 0.436 rad. Hence the sinusoidal expression for the resultant waveform is: yR = 6.1 sin(ωt − 0.436)

Adding of waveforms

6.1 25°

y

6

y1 ⫽ 4 sin vt

4

y2 ⫽ 3 sin(ωt⫺ π/3) yR⫽ y1 ⫹ y2

2 90° π/2

0 ⫺2

180° π

270° 3π/2

360° 2π

ωt

2. Two alternating voltages are given by v1 = 10 sin ωt volts and v2 = 14 sin (ωt + π/3) volts. By plotting v1 and v2 on the same axes over one cycle obtain a sinusoidal expression for (a) v1 + v2 (b) v1 − v2 3. Express 12 sin ωt + 5 cos ωt in the form A sin (ωt ± α) by drawing and measurement.

25

⫺4 ⫺6

28.3 Determining resultant phasors by calculation

Fig. 28.2 Problem 3. Determine a sinusoidal expression for y1 − y2 when y1 = 4 sin ωt and y2 = 3 sin (ωt − π/3)

45°

y 4 3.6

215

y1

y1 ⫺ y2 y2

2

0 ⫺2

90° π/2

180° π

270° 3π/2

360° 2π

The resultant of two periodic functions may be found from their relative positions when the time is zero. For example, if y1 = 4 sin ωt and y2 = 3 sin (ωt − π/3) then each may be represented as phasors as shown in Fig. 28.4, y1 being 4 units long and drawn horizontally and y2 being 3 units long, lagging y1 by π/3 radians or 60◦ . To determine the resultant of y1 + y2 , y1 is drawn horizontally as shown in Fig. 28.5 and y2 is joined to the end of y1 at 60◦ to the horizontal. The resultant is given by yR . This is the same as the diagonal of a parallelogram that is shown completed in Fig. 28.6. Resultant yR , in Figs. 28.5 and 28.6, is determined either by:

ωt y1 ⫽ 4

⫺4

60° or π/3 rads

0

y1 ⫽ 4 f

60°

a

y 2⫽ 3

Fig. 28.3 y1 and y2 are shown plotted in Fig. 28.3. At 15◦ intervals y2 is subtracted from y1 . For example: at 0◦ ,

y1 − y2 = 0 − (−2.6) = +2.6

at 30◦ ,

y1 − y2 = 2 − (−1.5) = +3.5

at 150◦ ,

y1 − y2 = 2 − 3 = −1, and so on.

yR b

y2 ⫽ 3

Fig. 28.5

Fig. 28.4

y1 ⫽ 4 f

The amplitude, or peak value of the resultant (shown by the broken line), is 3.6 and it leads y1 by 45◦ or 0.79 rad. Hence y1 − y2 = 3.6 sin(ωt + 0.79)

yR y2 ⫽ 3

Fig. 28.6

Now try the following exercise Exercise 101

Further problems on plotting periodic functions (Answers on page 281)

1. Plot the graph of y = 2 sin A from A = 0◦ to A = 360◦ . On the same axes plot y = 4 cos A. By adding ordinates at intervals plot y = 2 sin A + 4 cos A and obtain a sinusoidal expression for the waveform.

(a) use of the cosine rule (and then sine rule to calculate angle φ), or (b) determining horizontal and vertical components of lengths oa and ab in Fig. 28.5, and then using Pythagoras’ theorem to calculate ob. In the above example, by calculation, yR = 6.083 and angle φ = 25.28◦ or 0.441 rad. Thus the resultant may be expressed

216

Basic Engineering Mathematics

in sinusoidal form as yR = 6.083 sin(ωt − 0.441). If the resultant phasor, yR = y1 − y2 is required, then y2 is still 3 units long but is drawn in the opposite direction, as shown in Fig. 28.7, and yR is determined by calculation.

yR

y2 ⫽ 3

π/4 or 45° y1 ⫽ 2 (a)

⫺y2 ⫽ 3

yR y2 ⫽ 3

f

60° y1 ⫽ 4

60° 135°

f

45°

y1 ⫽ 2 (b) y2 ⫽ 3

y2

yR

Fig. 28.7 Resolution of phasors by calculation is demonstrated in worked problems 4 to 6. f

Problem 4. Given y1 = 2 sin ωt and y2 = 3 sin(ωt + π/4), obtain an expression for the resultant yR = y1 + y2 , (a) by drawing, and (b) by calculation

(a) When time t = 0 the position of phasors y1 and y2 are as shown in Fig. 28.8(a). To obtain the resultant, y1 is drawn horizontally, 2 units long, y2 is drawn 3 units long at an angle of π/4 rads or 45◦ and joined to the end of y1 as shown in Fig. 28.8(b). yR is measured as 4.6 units long and angle φ is measured as 27◦ or 0.47 rad. Alternatively, yR is the diagonal of the parallelogram formed as shown in Fig. 28.8(c).

y1 ⫽ 2

(c)

Fig. 28.8

Problem 5. Two alternating voltages are given by v1 = 15 sin ωt volts and v2 = 25 sin (ωt − π/6) volts. Determine a sinusoidal expression for the resultant vR = v1 + v2 by finding horizontal and vertical components. The relative positions of v1 and v2 at time t = 0 are shown in Fig. 28.9(a) and the phasor diagram is shown in Fig. 28.9(b).

Hence, by drawing, yR = 4.6 sin(ωt + 0.47)

v1 ⫽ 15 V π/6 or 30°

(b) From Fig. 28.8(b), and using the cosine rule: yR2 = 22 + 32 − [2(2)(3) cos 135◦ ] = 4 + 9 − [−8.485] = 21.49 Hence

√ yR = 21.49 = 4.64 0

4.64 3 = from Using the sine rule: sin φ sin 135◦ which sin φ =

v2 ⫽ 25 V

(a)

v1 a f

150°

b 30°

3 sin 135◦ = 0.4572 4.64

v2 vR c

Hence φ = sin−1 0.4572 = 27.21◦ or 0.475 rad. By calculation.

yR = 4.64 sin(ωt + 0.475)

(b)

Fig. 28.9

Adding of waveforms

The horizontal component of v R ,

vR

217

⫺v2 ⫽ 25 V

H = 15 cos 0◦ + 25 cos(−30◦ ) = oa + ab = 36.65 V f

The vertical component of v R ,

v1 ⫽ 15 V

V = 15 sin 0◦ + 25 sin(−30◦ ) Hence

30° 30°

= bc = −12.50 V vR (= oc) = 36.652 + (−12.50)2 by Pythagoras’ theorem = 38.72 volts V bc −12.50 tan φ = = = H ob 36.65 = −0.3411 φ = tan−1 (−0.3411)

from which,

= −18.83◦ or −0.329 radians. υR = υ1 + υ2

Hence

= 38.72 sin(ωt − 0.329) V Problem 6. For the voltages in Problem 5, determine the resultant vR = v1 − v2 To find the resultant vR = v1 − v2 , the phasor v2 of Fig. 28.9(b) is reserved in direction as shown in Fig. 28.10. Using the cosine rule: vR2 = 152 + 252 − 2(15)(25) cos 30◦ = 225 + 625 − 649.5 = 200.5 √ vR = 200.5 = 14.16 volts Using the sine rule: sin φ =

14.16 25 = from which sin φ sin 30◦

25 sin 30◦ = 0.8828 14.16

Hence φ = sin−1 0.8828 = 61.98◦ or 118.02◦ . From Fig. 28.10, φ is obtuse,

v2 ⫽ 25 V

Fig. 28.10 hence φ = 118.02◦ or 2.06 radians. Hence vR = v1 − v2 = 14.16 sin(ωt + 2.06) V Now try the following exercise Exercise 102

Further problems on the determination of resultant phasors by calculation (Answers on page 281)

In Problems 1 to 5, express the combination of periodic functions in the form A sin (ωt ± α) by calculation. π

1. 7 sin ωt + 5 sin ωt + 4 π

2. 6 sin ωt + 3 sin ωt − 6 π

A 3. i = 25 sin ωt − 15 sin ωt + 3 π

V 4. v = 8 sin ωt − 5 sin ωt − 4 3π π

− 7 sin ωt − 5. x = 9 sin ωt + cm 3 8 6. The currents in two parallel branches of an electrical circuit are: π

i1 = 5 sin ωt mA and i2 = 12 sin ωt + mA. 2 Determine the total current, iT , given that iT = i1 + i2

29 Number sequences

29.1 Simple sequences A set of numbers which are connected by a definite law is called a series or a sequence of numbers. Each of the numbers in the series is called a term of the series. For example, 1, 3, 5, 7, . . . is a series obtained by adding 2 to the previous term, and 2, 8, 32, 128, . . . is a sequence obtained by multiplying the previous term by 4.

Problem 1. Determine the next two terms in the series: 3, 6, 9, 12, . . . .

Now try the following exercise Exercise 103

Further problems on simple sequences (Answers on page 281)

Determine the next two terms in each of the following series: 1. 5, 9, 13, 17, . . .

2. 3, 6, 12, 24, . . .

3. 112, 56, 28, . . .

4. 12, 7, 2, . . .

5. 2, 5, 10, 17, 26, 37, . . .

6. 1, 0.1, 0.01, . . .

7. 4, 9, 19, 34, . . .

29.2 The n th term of a series We notice that the sequence 3, 6, 9, 12, . . . progressively increases by 3, thus the next two terms will be 15 and 18.

Problem 2. Find the next three terms in the series: 9, 5, 1, . . . .

We notice that each term in the series 9, 5, 1, . . . progressively decreases by 4, thus the next two terms will be 1 − 4, i.e. −3 and −3 − 4, i.e. −7.

Problem 3. Determine the next two terms in the series: 2, 6, 18, 54, . . . .

We notice that the second term, 6, is three times the first term, the third term, 18, is three times the second term, and that the fourth term, 54, is three times the third term. Hence the fifth term will be 3 × 54 = 162 and the sixth term will be 3 × 162 = 486

If a series is represented by a general expression, say, 2n + 1, where n is an integer (i.e. a whole number), then by substituting n = 1, 2, 3, . . . the terms of the series can be determined; in this example, the first three terms will be: 2(1) + 1, 2(2) + 1, 2(3) + 1, . . . ,

i.e. 3, 5, 7, . . .

What is the n th term of the sequence 1, 3, 5, 7, . . . ? Firstly, we notice that the gap between each term is 2, hence the law relating the numbers is: The second term,

‘2n + something’ 3 = 2n + something,

hence when n = 2 (i.e. the second term of the series), then 3 = 4 + something and the ‘something’ must be −1. Thus the n th term of 1, 3, 5, 7, . . . is 2n − 1. Hence the fifth term is given by 2(5) − 1 = 9, and the twentieth term is 2(20) − 1 = 39, and so on. Problem 4. The n th term of a sequence is given by 3n + 1. Write down the first four terms.

Number sequences

The first four terms of the series 3n + 1 will be: 3(1) + 1, 3(2) + 1, 3(3) + 1 i.e.

and

3(4) + 1

The first four terms on the series 4n − 1 will be: 4(1) − 1, 4(2) − 1, 4(3) − 1 i.e.

Now try the following exercise Exercise 104

4, 7, 10 and 13

Problem 5. The n th term of a series is given by 4n − 1. Write down the first four terms.

and

4(4) − 1

219

Further problems on the n the term of a series (Answers on page 281)

1. The n th term of a sequence is given by 2n − 1. Write down the first four terms. 2. The n th term of a sequence is given by 3n + 4. Write down the first five terms. 3. Write down the first four terms of the sequence given by 5n + 1 Find the n th term in the following series:

3, 7, 11 and 15

Problem 6. Find the n th term of the series: 1, 4, 7, . . . .

4. 5, 10, 15, 20, . . .

5. 4, 10, 16, 22, . . .

6. 3, 5, 7, 9, . . .

7. 2, 6, 10, 14, . . .

8. 9, 12, 15, 18, . . .

9. 1, 8, 27, 64, 125, . . .

We notice that the gap between each of the given three terms is 3, hence the law relating the numbers is: The second term,

‘3n + something’ 4 = 3n + something,

29.3 Arithmetic progressions

so the ‘something’ must be −2 (from simple equations).

When a sequence has a constant difference between successive terms it is called an arithmetic progression (often abbreviated to AP).

Thus the n th term of the series 1, 4, 7, . . . is: 3n − 2

Examples include:

so when n = 2, then 4 = 6 + something,

Problem 7. Find the n th term of the sequence: 3, 9, 15, 21, . . . . Hence determine the 15th term of the series. We notice that the gap between each of the given four terms is 6, hence the law relating the numbers is: The second term,

(i) 1, 4, 7, 10, 13, . . . where the common difference is 3, and (ii) a, a + d, a + 2d, a + 3d, . . . where the common difference is d. If the first term of an AP is ‘a’ and the common difference is ‘d’ then

‘6n + something’ 9 = 6n + something,

the n th term is : a + (n − 1)d

so when n = 2, then 9 = 12 + something, so the ‘something’ must be −3

Thus the n th term of the series 3, 9, 15, 21, . . . is: 6n − 3 The 15th term of the series is given by 6n − 3 when n = 15. Hence the 15th term of the series 3, 9, 15, 21, . . . is: 6(15) −3 = 87 Problem 8. 16, 25, . . . .

Find the n th term of the series: 1, 4, 9,

In example (i) above, the 7th term is given by 1 + (7 − 1)3 = 19, which may be readily checked. The sum S of an AP can be obtained by multiplying the average of all the terms by the number of terms. a+1 , where ‘a’ is the first term 2 and l is the last term, i.e. l = a + (n − 1)d, for n terms.

The average of all the terms = Hence the sum of n terms, Sn = n

This is a special series and does not follow the pattern of the previous examples. Each of the terms in the given series are square numbers, i.e.

a+1 2

=

n {a + [a + (n − 1)d]} 2

1, 4, 9, 16, 25, . . . ≡ 12 , 22 , 32 , 42 , 52 , . . .

Hence the n th term is: n2

i.e.

n Sn = [2a + (n − 1)d] 2

220

Basic Engineering Mathematics

For example, the sum of the first 7 terms of the series 1, 4, 7, 10, 13, . . . is given by S7 = 72 [2(1) + (7 − 1)3],

i.e.

1 = 22 2 1 1 1 = 22 − 2 = 19 (n − 1) 1 2 2 2

1 2 + (n − 1) 2

since a = 1 and d = 3

= 72 [2 + 18] = 72 [20] = 70

n−1=

1

19 12 1 12

= 13 and n = 13 + 1 = 14

i.e. the 14th term of the AP is 22

29.4 Worked problems on arithmetic progression

Problem 12. Find the sum of the first 12 terms of the series 5, 9, 13, 17, . . . . Problem 9. Determine (a) the ninth, and (b) the sixteenth term of the series 2, 7, 12, 17, . . . .

2, 7, 12, 17, . . . is an arithmetic progression with a common difference, d, of 5

5, 9, 13, 17, . . . is an AP where a = 5 and d = 4 The sum of n terms of an AP, Sn =

(a) The n th term of an AP is given by a + (n − 1)d

n [2a + (n − 1)d] 2

Hence the sum of the first 12 terms,

Since the first term a = 2, d = 5 and n = 9

12 [2(5) + (12 − 1)4] 2 = 6[10 + 44] = 6(54) = 324

then the 9th term is:

S12 =

2 + (9 − 1)5 = 2 + (8)(5) = 2 + 40 = 42 (b) The 16th term is:

Problem 13. Find the sum of the first 21 terms of the series 3.5, 4.1, 4.7, 5.3, . . . .

2 + (16 − 1)5 = 2 + (15)(5) = 2 + 75 = 77

Problem 10. The 6th term of an AP is 17 and the 13th term is 38. Determine the 19th term.

3.5, 4.1, 4.7, 5.3, . . . is an AP where a = 3.5 and d = 0.6 The sum of the first 21 terms, S21 =

The n th term of an AP is a + (n − 1)d The 6th term is:

a + 5d = 17

(1)

The 13th term is:

a + 12d = 38

(2)

Equation (2) − equation (1) gives: 7d = 21, from which, 21 d= =3 7 Substituting in equation (1) gives: a + 15 = 17, from which, a = 2 Hence the 19th term is:

21 [2a + (n − 1)d] 2

=

21 21 [2(3.5) + (21 − 1)0.6] = [7 + 12] 2 2

=

399 21 (19) = = 199.5 2 2

Now try the following exercise

a + (n − 1)d = 2 + (19 − 1)3 = 2 + (18)(3) = 2 + 54 = 56 Problem 11. Determine the number of the term whose value is 22 in the series 2 12 , 4, 5 12 , 7, . . . 1 1 1 1 2 , 4, 5 , 7, . . . is an AP where a = 2 and d = 1 2 2 2 2

Hence if the n th term is 22 then: a + (n − 1)d = 22

Exercise 105

Further problems on arithmetic progressions (Answers on page 281)

1. Find the 11th term of the series 8, 14, 20, 26, . . . . 2. Find the 17th term of the series 11, 10.7, 10.4, 10.1, . . . . 3. The seventh term of a series is 29 and the eleventh term is 54. Determine the sixteenth term. 4. Find the 15th term of an arithmetic progression of which the first term is 2 12 and the tenth term is 16.

Number sequences

5. Determine the number of the term which is 29 in the series 7, 9.2, 11.4, 13.6, . . . . 6. Find the sum of the first 11 terms of the series 4, 7, 10, 13, . . . . 7. Determine the sum of the series 6.5, 8.0, 9.5, 11.0, . . ., 32

29.5 Further worked problems on arithmetic progressions Problem 14. The sum of 7 terms of an AP is 35 and the common difference is 1.2. Determine the first term of the series. n = 7, d = 1.2 and S7 = 35 Since the sum of n terms of an AP is given by n Sn = [2a + (n − 1) d], then 2 7 7 35 = [2a + (7 − 1)1.2] = [2a + 7.2] 2 2 35 × 2 = 2a + 7.2 Hence 7 10 = 2a + 7.2 Thus 2a = 10 − 7.2 = 2.8, from which a =

2.8 = 1.4 2

i.e. the first term, a = 1.4 Problem 15. Three numbers are in arithmetic progression. Their sum is 15 and their product is 80. Determine the three numbers. Let the three numbers be (a − d), a and (a + d) Then (a − d) + a + (a + d) = 15, i.e. 3a = 15, from which, a = 5 Also, a(a − d)(a + d) = 80, i.e. a(a2 − d 2 ) = 80 Since

a = 5, 5(52 − d 2 ) = 80 125 − 5d 2 = 80 125 − 80 = 5d 2 45 = 5d 2

√ 45 = 9. Hence d = 9 = ± 3 5 The three numbers are thus (5 − 3), 5 and (5 + 3), i.e. 2, 5 and 8 from which, d 2 =

The last term is a + (n − 1)d = 207 3 + (n − 1)3 = 207, from which 207 − 3 (n − 1) = = 68 3 Hence n = 68 + 1 = 69 i.e.

The sum of all 69 terms is given by n S69 = [2a + (n − 1)d] 2 69 = [2(3) + (69 − 1)3] 2 69 69 = [6 + 204] = (210) = 7245 2 2 Problem 17. The first, twelfth and last term of an arithmetic progression are 4, 31 12 , and 376 12 respectively. Determine (a) the number of terms in the series, (b) the sum of all the terms and (c) the 80’th term. (a) Let the AP be a, a + d, a + 2d, . . . , a + (n − 1)d, where a = 4 The 12th term is: a + (12 − 1)d = 31 12 i.e.

4 + 11d = 31 12 , from which, 11d = 31 12 − 4 = 27 12

Hence d =

27 12 1 =2 11 2

The last term is a + (n − 1)d i.e. 4 + (n − 1) 2 12 = 376 12 (n − 1) =

376 12 − 4 2 12

=

The series 3, 6, 9, 12, . . . 207 is an AP whose first term a = 3 and common difference d = 3

372 12 2 12

= 149

Hence the number of terms in the series, n = 149 + 1 = 150 (b) Sum of all the terms, n S150 = [2a + (n − 1)d] 2 1 150 2(4) + (150 − 1) 2 = 2 2 1 = 75[8 + 372.5] = 75 8 + (149) 2 2 = 75(380.5) = 28537.5 (c) The 80th term is:

Problem 16. Find the sum of all the numbers between 0 and 207 which are exactly divisible by 3.

221

a + (n − 1)d = 4 + (80 − 1) 2 12 = 4 + (79) 2 12 = 4 + 197.5 = 201 12

222

Basic Engineering Mathematics

then the sum of n terms,

Now try the following exercise Exercise 106

Further problems on arithmetic progressions (Answers on page 281)

1. The sum of 15 terms of an arithmetic progression is 202.5 and the common difference is 2. Find the first term of the series.

Sn = a + ar + ar 2 + ar 3 + · · · + ar n−1 · · · Multiplying throughout by r gives: rSn = ar + ar 2 + ar 3 + ar 4 + · · · ar n−1 + ar n · · ·

Sn − rSn = a − ar n i.e.

3. Find the sum of all the numbers between 5 and 250 which are exactly divisible by 4.

Thus the sum of n terms,

4. Find the number of terms of the series 5, 8, 11, . . . of which the sum is 1025.

when r < 1

5. Insert four terms between 5 and metic progression.

to form an arith-

6. The first, tenth and last terms of an arithmetic progression are 9, 40.5, and 425.5 respectively. Find (a) the number of terms, (b) the sum of all the terms and (c) the 70th term. 7. On commencing employment a man is paid a salary of £7200 per annum and receives annual increments of £350. Determine his salary in the 9th year and calculate the total he will have received in the first 12 years. 8. An oil company bores a hole 80 m deep. Estimate the cost of boring if the cost is £30 for drilling the first metre with an increase in cost of £2 per metre for each succeeding metre.

(2)

Subtracting equation (2) from equation (1) gives:

2. Three numbers are in arithmetic progression. Their sum is 9 and their product is 20 14 . Determine the three numbers.

22 12

(1)

Sn (1 − r) = a(1 − r n ) Sn =

a(1 − r n ) (1 − r)

which is valid

Subtracting equation (1) from equation (2) gives Sn =

a(r n − 1) (r − 1)

which is valid when r > 1

For example, the sum of the first 8 terms of the GP 1, 2, 4, 8, 16, . . . is given by 1(28 − 1) , since a = 1 and r = 2 (2 − 1) 1(256 − 1) = 255 i.e. S8 = 1 When the common ratio r of a GP is less than unity, the sum of n terms, S8 =

a(1 − r n ) , which may be written as (1 − r) ar n a Sn = − (1 − r) (1 − r) Sn =

29.6 Geometric progressions When a sequence has a constant ratio between successive terms it is called a geometric progression (often abbreviated to GP). The constant is called the common ratio, r Examples include (i) 1, 2, 4, 8, . . . where the common ratio is 2, and (ii) a, ar, ar 2 , ar 3 , . . . where the common ratio is r

Since r < 1, r n becomes less as n increases, rn → 0

i.e.

as

n→∞

n

ar → 0 as n → ∞ (1 − r) a Thus Sn → as n → ∞ (1 − r) a is called the sum to infinity, S∞ , and is the The quantity (1 − r) limiting value of the sum of an infinite number of terms, Hence

If the first term of a GP is ‘a’ and the common ratio is r, then the n th term is : ar n−1

i.e.

S∞ =

a (1−r)

which is valid when −1 < r < 1

which can be readily checked from the above examples.

For example, the sum to infinity of the GP 1, 12 , 14 , . . . is

For example, the 8th term of the GP 1, 2, 4, 8, . . . is (1)(2)7 = 128, since a = 1 and r = 2

S∞ =

Let a GP be a, ar, ar 2 , ar 3 , . . . ar n−1

1 , since a = 1 and r = 12 , i.e. S∞ = 2 1 − 12

Number sequences

29.7 Worked problems on geometric progressions Problem 18. 12, 24, . . . .

Hence

Determine the tenth term of the series 3, 6,

223

n−1 1 1 = (2187) from which 9 3 9 n−1 1 1 1 1 1 = 2 7 = 9 = = 3 (9)(2187) 3 3 3 3

Thus (n − 1) = 9, from which, n = 9 + 1 = 10 3, 6, 12, 24, . . . is a geometric progression with a common ratio r of 2 The n th term of a GP is ar n−1 , where a is the first term. Hence the 10th term is:

i.e.

1 is the 10th term of the GP 9

Problem 22. Find the sum of the first 9 terms of the series 72.0, 57.6, 46.08, . . . .

(3)(2)10−1 = (3)(2)9 = 3(512) = 1536 The common ratio, Problem 19. Find the sum of the first 7 terms of the series, 1 , 1 12 , 4 12 , 13 12 , . . . . 2

r=

57.6 ar 2 46.08 ar = = 0.8 also = = 0.8 a 72.0 ar 57.6

The sum of 9 terms, 1 , 1 12 , 4 12 , 13 12 , . . . 2

is a GP with a common ratio r = 3

The sum of n terms, Sn = Hence S7 =

a(r n − 1) (r − 1)

1 − 1) (2187 − 1) = 2 = 546 12 (3 − 1) 2

a(1 − r n ) 72.0(1 − 0.89 ) = (1 − r) (1 − 0.8) 72.0(1 − 0.1342) = 311.7 = 0.2

S9 =

1 7 (3 2

Problem 20. The first term of a geometric progression is 12 and the fifth term is 55. Determine the 8th term and the 11th term.

The 5th term is given by ar 4 = 55, where the first term a = 12 55 55 55 Hence r 4 = = and r = 4 = 1.4631719 . . . . a 12 12

Problem 23. 1, 13 , . . . .

Find the sum to infinity of the series 3,

3, 1, 13 , . . . is a GP of common ratio, r =

1 3

The sum to infinity, S∞ =

3 a = 1−r 1−

1 3

=

3 2 3

=

1 9 =4 2 2

Now try the following exercise

The 8th term is ar 7 = (12)(1.4631719 . . . )7 = 172.3 The 11th term is

Further problems on geometric progressions (Answers on page 281)

1. Find the 10th term of the series 5, 10, 20, 40, . . . .

ar = (12)(1.4631719 . . . ) = 539.7 10

Exercise 107

10

Problem 21. Which term of the series 2187, 729, 243, . . . is 19 ?

2187, 729, 243, . . . is a GP with a common ratio r = 13 and first term a = 2187 The n th term of a GP is given by: ar n−1

2. Determine the sum of the first 7 terms of the series 1 3 , , 2 14 , 6 34 , . . . . 4 4 3. The first term of a geometric progression is 4 and the 6th term is 128. Determine the 8th and 11th terms. 4. Which term of the series 3, 9, 27, . . . is 59049? 5. Find the sum of the first 7 terms of the series 2, 5, 12 12 , . . . (correct to 4 significant figures). 6. Determine the sum to infinity of the series 4, 2, 1, . . . . 7. Find the sum to infinity of the series 2 12 , −1 14 , 58 , . . . .

224

Basic Engineering Mathematics

29.8 Further worked problems on geometric progressions Problem 24. In a geometric progression the sixth term is 8 times the third term and the sum of the seventh and eighth terms is 192. Determine (a) the common ratio, (b) the first term, and (c) the sum of the fifth to eleventh terms, inclusive. 2

3

(a) Let the GP be a, ar, ar , ar , . . . , ar

n−1

The 3rd term = ar 2 and the sixth term = ar 5 The 6th term is 8 times the 3rd

√ Hence ar 5 = 8ar 2 from which, r 3 = 8, r = 3 8

i.e. the common ratio r = 2 (b) The sum of the 7th and 8th terms is 192. Hence ar 6 + ar 7 = 192 Since r = 2, then 64a + 128a = 192 192a = 192 from which, a, the first term = 1

(b) the time, correct to the nearest year, it takes to reach more than £300. (a) Let the GP be a, ar, ar 2 , . . . ar n The first term a = £100 The common ratio r = 1.08 Hence the second term is ar = (100)(1.08) = £108, which is the value after 1 year, the third term is ar 2 = (100)(1.08)2 = £116.64, which is the value after 2 years, and so on. Thus the value after 10 years = ar 10 = (100)(1.08)10 = £215.89 (b) When £300 has been reached, 300 = ar n i.e.

300 = 100(1.08)n

and

3 = (1.08)n

Taking logarithms to base 10 of both sides gives: lg 3 = lg(1.08)n = n lg(1.08), by the laws of logarithms from which, n=

lg 3 = 14.3 lg 1.08

(c) The sum of the 5th to 11th terms (inclusive) is given by: S11 − S4 =

a(r 11 − 1) a(r 4 − 1) − (r − 1) (r − 1)

1(211 − 1) 1(24 − 1) − = (2 − 1) (2 − 1) = (211 − 1) − (24 − 1) = 211 − 24 = 2408 − 16 = 2032

Hence it will take 15 years to reach more than £300 Problem 27. A drilling machine is to have 6 speeds ranging from 50 rev/min to 750 rev/min. If the speeds form a geometric progression determine their values, each correct to the nearest whole number. Let the GP of n terms be given by a, ar, ar 2 , . . . ar n−1 The first term a = 50 rev/min.

Problem 25. A hire tool firm finds that their net return from hiring tools is decreasing by 10% per annum. If their net gain on a certain tool this year is £400, find the possible total of all future profits from this tool (assuming the tool lasts for ever). The net gain forms a series: £400 + £400 × 0.9 + £400 × 0.92 + · · · ,

The 6th term is given by ar 6−1 , which is 750 rev/min, i.e., ar 5 = 750 750 750 = = 15 from which r 5 = a 50 √ Thus the common ratio, r = 5 15 = 1.7188 The first term is a = 50 rev/min the second term is ar = (50)(1.7188) = 85.94,

which is a GP with a = 400 and r = 0.9

the third term is ar 2 = (50)(1.7188)2 = 147.71,

The sum to infinity, 400 a = S∞ = (1 − r) (1 − 0.9) = £4000 = total future profits

the fourth term is ar 3 = (50)(1.7188)3 = 253.89,

Problem 26. If £100 is invested at compound interest of 8% per annum, determine (a) the value after 10 years,

the fifth term is ar 4 = (50)(1.7188)4 = 436.39, the sixth term is ar 5 = (50)(1.7188)5 = 750.06 Hence, correct to the nearest whole number, the 6 speeds of the drilling machine are 50, 86, 148, 254, 436 and 750 rev/min.

Number sequences

Now try the following exercise Exercise 108

225

F1 ⫽ 10 N

Further problems on geometric progressions (Answers on page 281)

1. In a geometric progression the 5th term is 9 times the 3rd term and the sum of the 6th and 7th terms is 1944. Determine (a) the common ratio, (b) the first term and (c) the sum of the 4th to 10th terms inclusive.

60° 45°

F2 ⫽ 16 N

2. The value of a lathe originally valued at £3000 depreciates 15% per annum. Calculate its value after 4 years. The machine is sold when its value is less than £550. After how many years is the lathe sold? 3. If the population of Great Britain is 55 million and is decreasing at 2.4% per annum, what will be the population in 5 years time? 4. 100 g of a radioactive substance disintegrates at a rate of 3% per annum. How much of the substance is left after 11 years? 5. If £250 is invested at compound interest of 6% per annum determine (a) the value after 15 years, (b) the time, correct to the nearest year, it takes to reach £750 6. A drilling machine is to have 8 speeds ranging from 100 rev/min to 1000 rev/min. If the speeds form a geometric progression determine their values, each correct to the nearest whole number.

F3 ⫽ 20 N

Fig. A13.1

Plot the two voltages on the same axes to scales of 1 cm = 50 volts and 1 cm =π/6. Obtain a sinusoidal expression for the resultant of v1 and v2 in the form R sin (ωt + α): (a) by adding ordinates at intervals, and (b) by calculation. (15)

Assignment 13

5. Determine the 20th term of the series 15.6, 15, 14.4, 13.8, . . . . (3)

This assignment covers the material in chapters 27 to 29. The marks for each question are shown in brackets at the end of each question.

6. The sum of 13 terms of an arithmetic progression is 286 and the common difference is 3. Determine the first term of the series. (4)

1. Forces of 10 N, 16 N and 20 N act as shown in Figure A13.1. Determine the magnitude of the resultant force and its direction relative to the 16 N force (a) by scaled drawing, and (b) by calculation. (9) 2. For the three forces shown in Figure A13.1, calculate the resultant of F1 − F2 − F3 and its direction relative to force F2 (6) 3. Two cars, A and B, are travelling towards crossroads. A has a velocity of 60 km/h due south and B a velocity of 75 km/h due west. Calculate the velocity of A relative to B. (6) 4. The instantaneous values of two alternating voltages are given by: π

volts v1 = 150 sin ωt + 3 π

and v2 = 90 sin ωt − volts 6

7. An engineer earns £21000 per annum and receives annual increments of £600. Determine the salary in the 9th year and calculate the total earnings in the first 11 years. (5) 8. Determine the 11th term of the series 1.5, 3, 6, 12, . . . . (2) 9. Find the sum of the first eight terms of the series 1, 2 12 , 6 14 , . . . , correct to 1 decimal place. (4) 10. Determine the sum to infinity of the series 5, 1, 15 , . . . . (3) 11. A machine is to have seven speeds ranging from 25 rev/min to 500 rev/min. If the speeds form a geometric progression, determine their value, each correct to the nearest whole number. (8)

30 Presentation of statistical data

30.1 Some statistical terminology Data are obtained largely by two methods: (a) by counting – for example, the number of stamps sold by a post office in equal periods of time, and (b) by measurement – for example, the heights of a group of people. When data are obtained by counting and only whole numbers are possible, the data are called discrete. Measured data can have any value within certain limits and are called continuous (see Problem 1). A set is a group of data and an individual value within the set is called a member of the set. Thus, if the masses of five people are measured correct to the nearest 0.1 kilogram and are found to be 53.1 kg, 59.4 kg, 62.1 kg, 77.8 kg and 64.4 kg, then the set of masses in kilograms for these five people is: {53.1, 59.4, 62.1, 77.8, 64.4} and one of the members of the set is 59.4. A set containing all the members is called a population. Some members selected at random from a population are called a sample. Thus all car registration numbers form a population, but the registration numbers of, say, 20 cars taken at random throughout the country are a sample drawn from that population. The number of times that the value of a member occurs in a set is called the frequency of that member. Thus in the set: {2, 3, 4, 5, 4, 2, 4, 7, 9}, member 4 has a frequency of three, member 2 has a frequency of 2 and the other members have a frequency of one. The relative frequency with which any member of a set occurs is given by the ratio: frequency of member total frequency of all members For the set: {2, 3, 5, 4, 7, 5, 6, 2, 8}, the relative frequency of member 5 is 29

Often, relative frequency is expressed as a percentage and the percentage relative frequency is: (relative frequency × 100)% Problem 1. Data are obtained on the topics given below. State whether they are discrete or continuous data. (a) The number of days on which rain falls in a month for each month of the year. (b) The mileage travelled by each of a number of salesmen. (c) The time that each of a batch of similar batteries lasts. (d) The amount of money spent by each of several families on food. (a) The number of days on which rain falls in a given month must be an integer value and is obtained by counting the number of days. Hence, these data are discrete. (b) A salesman can travel any number of miles (and parts of a mile) between certain limits and these data are measured. Hence the data are continuous. (c) The time that a battery lasts is measured and can have any value between certain limits. Hence these data are continuous. (d) The amount of money spent on food can only be expressed correct to the nearest pence, the amount being counted. Hence, these data are discrete.

Now try the following exercise Exercise 109

Further problems on discrete and continuous data (Answers on page 281)

In Problems 1 and 2, state whether data relating to the topics given are discrete or continuous.

Presentation of statistical data

1. (a) The amount of petrol produced daily, for each of 31 days, by a refinery.

Month

February

(c) The number of bottles of milk delivered daily by each of 20 milkmen.

April

March

May

(d) The size of 10 samples of rivets produced by a machine.

(b) The time taken by each of 12 athletes to run 100 metres. (c) The value of stamps sold in a day by each of 20 post offices. (d) The number of defective items produced in each of 10 one-hour periods by a machine.

June

Fig. 30.1 11 sets repaired, in February, 3 symbols are used to represent the 6 sets repaired, and so on. Problem 3. The distance in miles travelled by four salesmen in a week are as shown below. Salesmen Distance traveled (miles)

30.2 Presentation of ungrouped data Ungrouped data can be presented diagrammatically in several ways and these include: (a) pictograms, in which pictorial symbols are used to represent quantities (see Problem 2), (b) horizontal bar charts, having data represented by equally spaced horizontal rectangles (see Problem 3), and

⬅2 sets

January

(b) The amount of coal produced daily by each of 15 miners.

2. (a) The number of people visiting an exhibition on each of 5 days.

Number of TV sets repaired

227

P 413

Q 264

R 597

S 143

Use a horizontal bar chart to represent these data diagrammatically.

Equally spaced horizontal rectangles of any width, but whose length is proportional to the distance travelled, are used. Thus, the length of the rectangle for salesman P is proportional to 413 miles, and so on. The horizontal bar chart depicting these data is shown in Fig. 30.2.

Trends in ungrouped data over equal periods of time can be presented diagrammatically by a percentage component bar chart. In such a chart, equally spaced rectangles of any width, but whose height corresponds to 100%, are constructed. The rectangles are then subdivided into values corresponding to the percentage relative frequencies of the members (see Problem 5). A pie diagram is used to show diagrammatically the parts making up the whole. In a pie diagram, the area of a circle represents the whole, and the areas of the sectors of the circle are made proportional to the parts which make up the whole (see Problem 6). Problem 2. The number of television sets repaired in a workshop by a technician in six, one-month periods is as shown below. Present these data as a pictogram. Month January February March April May June Number repaired 11 6 15 9 13 8

Each symbol shown in Fig. 30.1 represents two television sets repaired. Thus, in January, 5 12 symbols are used to represent the

Salesmen

(c) vertical bar charts, in which data are represented by equally spaced vertical rectangles (see Problem 4). S R Q P 0

100

200 300 400 500 Distance travelled, miles

600

Fig. 30.2

Problem 4. The number of issues of tools or materials from a store in a factory is observed for seven, one-hour periods in a day, and the results of the survey are as follows: Period Number of issues

1 34

2 17

3 9

4 5

5 27

6 13

7 6

Present these data on a vertical bar chart.

In a vertical bar chart, equally spaced vertical rectangles of any width, but whose height is proportional to the quantity being

Basic Engineering Mathematics

Number of issues

represented, are used. Thus the height of the rectangle for period 1 is proportional to 34 units, and so on. The vertical bar chart depicting these data is shown in Fig. 30.3.

40

the three years. The heights of the rectangles correspond to 100% relative frequency, and are subdivided into the values in the table of percentages shown above. A key is used (different types of shading or different colour schemes) to indicate corresponding percentage values in the rows of the table of percentages. The percentage component bar chart is shown in Fig. 30.4.

30 Key

20 10 1

2

3

4

5

6

7

Periods

Fig. 30.3

Problem 5. The numbers of various types of dwellings sold by a company annually over a three-year period are as shown below. Draw percentage component bar charts to present these data.

4-roomed bungalows 5-roomed bungalows 4-roomed houses 5-roomed houses 6-roomed houses

Year 1

Year 2

Year 3

24 38 44 64 30

17 71 50 82 30

7 118 53 147 25

A table of percentage relative frequency values, correct to the nearest 1%, is the first requirement. Since, percentage relative frequency frequency of member × 100 = total frequency then for 4-roomed bungalows in year 1: 24 × 100 24 + 38 + 44 + 64 + 30 = 12%

percentage relative frequency =

The percentage relative frequencies of the other types of dwellings for each of the three years are similarly calculated and the results are as shown in the table below.

4-roomed bungalows 5-roomed bungalows 4-roomed houses 5-roomed houses 6-roomed houses

Year 1

Year 2

Year 3

12% 19% 22% 32% 15%

7% 28% 20% 33% 12%

2% 34% 15% 42% 7%

The percentage component bar chart is produced by constructing three equally spaced rectangles of any width, corresponding to

Percentage relative frequency

228

100

6-roomed houses

90

5-roomed houses

80

4-roomed houses

70

5-roomed bungalows

60

4-roomed bungalows

50 40 30 20 10 1

2 Year

3

Fig. 30.4

Problem 6. The retail price of a product costing £2 is made up as follows: materials 10p, labour 20p, research and development 40p, overheads 70p, profit 60p. Present these data on a pie diagram.

A circle of any radius is drawn, and the area of the circle represents the whole, which in this case is £2. The circle is subdivided into sectors so that the areas of the sectors are proportional to the parts, i.e. the parts which make up the total retail price. For the area of a sector to be proportional to a part, the angle at the centre of the circle must be proportional to that part. The whole, £2 or 200p, corresponds to 360◦ . Therefore, 10p corresponds to 360 ×

10 degrees, i.e. 18◦ 200

20p corresponds to 360 ×

20 degrees, i.e. 36◦ 200

and so on, giving the angles at the centre of the circle for the parts of the retail price as: 18◦ , 36◦ , 72◦ , 126◦ and 108◦ , respectively. The pie diagram is shown in Fig. 30.5. Problem 7. (a) Using the data given in Fig. 30.2 only, calculate the amount of money paid to each salesman for travelling expenses, if they are paid an allowance of 37p per mile.

Presentation of statistical data

(b) Using the data presented in Fig. 30.4, comment on the housing trends over the three-year period. (c) Determine the profit made by selling 700 units of the product shown in Fig. 30.5

229

Now try the following exercise Exercise 110

Further problems on presentation of ungrouped data (Answers on page 282)

1. The number of vehicles passing a stationary observer on a road in six ten-minute intervals is as shown. Draw a pictogram to represent these data.

Research and development Labour 72° 36° 8° Materials 126° 108° Overheads Profit

Period of Time Number of Vehicles

1

2

3

4

5

6

35

44

62

68

49

41

2. The number of components produced by a factory in a week is as shown below: Day Mon Tues Wed Thur Fri Number of Components 1580 2190 1840 2385 1280

lp ⬅ 1.8°

Fig. 30.5 Show these data on a pictogram. (a) By measuring the length of rectangle P the mileage covered by salesman P is equivalent to 413 miles. Hence salesman P receives a travelling allowance of £413 × 37 100

i.e.

£152.81

Similarly, for salesman Q, the miles travelled are 264 and his allowance is £264 × 37 100

i.e.

£97.68

Salesman R travels 597 miles and he receives £597 × 37 100

i.e.

£220.89

3. For the data given in Problem 1 above, draw a horizontal bar chart. 4. Present the data given in Problem 2 above on a horizontal bar chart. 5. For the data given in Problem 1 above, construct a vertical bar chart. 6. Depict the data given in Problem 2 above on a vertical bar chart. 7. A factory produces three different types of components. The percentages of each of these components produced for three, one-month periods are as shown below. Show this information on percentage component bar charts and comment on the changing trend in the percentages of the types of component produced.

Finally, salesman S receives £143 × 37 100

i.e.

£52.91

(b) An analysis of Fig. 30.4 shows that 5-roomed bungalows and 5-roomed houses are becoming more popular, the greatest change in the three years being a 15% increase in the sales of 5-roomed bungalows. (c) Since 1.8◦ corresponds to 1p and the profit occupies 108◦ of the pie diagram, then the profit per unit is 108 × 1 1.8

i.e.

60p

The profit when selling 700 units of the product is £

700 × 60 100

i.e. £420

Month

1

2

3

Component P Component Q Component R

20 45 35

35 40 25

40 35 25

8. A company has five distribution centres and the mass of goods in tonnes sent to each centre during four, one-week periods, is as shown. Week Centre A Centre B Centre C Centre D Centre E

1

2

3

4

147 54 283 97 224

160 63 251 104 218

174 77 237 117 203

158 69 211 144 194

Use a percentage component bar chart to present these data and comment on any trends.

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Basic Engineering Mathematics

9. The employees in a company can be split into the following categories: managerial 3, supervisory 9, craftsmen 21, semi-skilled 67, others 44. Show these data on a pie diagram.

Lower Class Upper class mid-point class boundary boundary

10. The way in which an apprentice spent his time over a one-month period is as follows: drawing office 44 hours, production 64 hours, training 12 hours, at college 28 hours.

to 7.3

(b) If in year 2 of Fig. 30.4, 1% corresponds to 2.5 dwellings, how many bungalows are sold in that year. 12. (a) If the company sell 23 500 units per annum of the product depicted in Fig. 30.5, determine the cost of their overheads per annum. (b) If 1% of the dwellings represented in year 1 of Fig. 30.4 corresponds to 2 dwellings, find the total number of houses sold in that year.

30.3 Presentation of grouped data When the number of members in a set is small, say ten or less, the data can be represented diagrammatically without further analysis, by means of pictograms, bar charts, percentage components bar charts or pie diagrams (as shown in Section 30.2). For sets having more than ten members, those members having similar values are grouped together in classes to form a frequency distribution. To assist in accurately counting members in the various classes, a tally diagram is used (see Problems 8 and 12). A frequency distribution is merely a table showing classes and their corresponding frequencies (see Problems 8 and 12). The new set of values obtained by forming a frequency distribution is called grouped data. The terms used in connection with grouped data are shown in Fig. 30.6(a). The size or range of a class is given by the upper class boundary value minus the lower class boundary value, and in Fig. 30.6 is 7.65 − 7.35, i.e. 0.30. The class interval for the class shown in Fig. 30.6(b) is 7.4 to 7.6 and the class mid-point value is given by (upper class boundary value) + (lower class boundary value) 2 7.65 + 7.35 , i.e. 7.5 2 One of the principal ways of presenting grouped data diagrammatically is by using a histogram, in which the areas and in Fig. 30.6 is

7.4 to 7.6

7.7 to

(b)

Use a pie diagram to depict this information. 11. (a) With reference to Fig. 30.5, determine the amount spent on labour and materials to produce 1650 units of the product.

Class interval

(a)

7.35

7.5

7.65

Fig. 30.6 of vertical, adjacent rectangles are made proportional to frequencies of the classes (see Problem 9). When class intervals are equal, the heights of the rectangles of a histogram are equal to the frequencies of the classes. For histograms having unequal class intervals, the area must be proportional to the frequency. Hence, if the class interval of class A is twice the class interval of class B, then for equal frequencies, the height of the rectangle representing A is half that of B (see Problem 11). Another method of presenting grouped data diagrammatically is by using a frequency polygon, which is the graph produced by plotting frequency against class mid-point values and joining the co-ordinates with straight lines (see Problem 12). A cumulative frequency distribution is a table showing the cumulative frequency for each value of upper class boundary. The cumulative frequency for a particular value of upper class boundary is obtained by adding the frequency of the class to the sum of the previous frequencies. A cumulative frequency distribution is formed in Problem 13. The curve obtained by joining the co-ordinates of cumulative frequency (vertically) against upper class boundary (horizontally) is called an ogive or a cumulative frequency distribution curve (see Problem 13).

Problem 8. The data given below refer to the gain of each of a batch of 40 transistors, expressed correct to the nearest whole number. Form a frequency distribution for these data having seven classes. 81 86 84 81 83

83 76 81 79 79

87 77 80 78 80

74 71 81 80 83

76 86 73 85 82

89 85 89 77 79

82 87 82 84 80

84 88 79 78 77

The range of the data is the value obtained by taking the value of the smallest member from that of the largest member. Inspection of the set of data shows that, range = 89 − 71 = 18. The size of each class is given approximately by range divided by the number of classes. Since 7 classes are required, the size of each class is 18/7, that is, approximately 3. To achieve

seven equal classes spanning a range of values from 71 to 89, the class intervals are selected as: 70–72, 73–75, and so on. To assist with accurately determining the number in each class, a tally diagram is produced, as shown in Table 30.1(a). This is obtained by listing the classes in the left-hand column, and then inspecting each of the 40 members of the set in turn and allocating them to the appropriate classes by putting ‘1s’ in the appropriate rows. Every fifth ‘1’ allocated to a particular row is shown as an oblique line crossing the four previous ‘1s’, to help with final counting. A frequency distribution for the data is shown in Table 30.1(b) and lists classes and their corresponding frequencies, obtained from the tally diagram. (Class mid-point values are also shown in the table, since they are used for constructing the histogram for these data (see Problem 9)). Table 30.1(a) Class 70–72 73–75 76–78 79–81 82–84 85–87 88–90

Tally 1 11 11 1111 11 1111 1111 1111 1111 1 1111 111

Table 30.1(b) Class

Class mid-point

Frequency

70–72 73–75 76–78 79–81 82–84 85–87 88–90

71 74 77 80 83 86 89

1 2 7 12 9 6 3

Frequency

Presentation of statistical data

16 14 12 10 8 6 4 2 71

74 77 80 83 86 Class mid-point values

89

Fig. 30.7

Problem 10. The amount of money earned weekly by 40 people working part-time in a factory, correct to the nearest £10, is shown below. Form a frequency distribution having 6 classes for these data. 80 140 80 130 50

90 30 90 170 100

70 90 110 80 110

110 50 80 120 90

90 100 100 100 100

160 110 90 110 70

110 60 120 40 110

80 100 70 110 80

Inspection of the set given shows that the majority of the members of the set lie between £80 and £110 and that there are a much smaller number of extreme values ranging from £30 to £170. If equal class intervals are selected, the frequency distribution obtained does not give as much information as one with unequal class intervals. Since the majority of members are between £80 and £100, the class intervals in this range are selected to be smaller than those outside of this range. There is no unique solution and one possible solution is shown in Table 30.2. Table 30.2 Class 20–40 50–70 80–90 100–110 120–140 150–170

Problem 9. Construct a histogram for the data given in Table 30.1(b).

The histogram is shown in Fig. 30.7. The width of the rectangles correspond to the upper class boundary values minus the lower class boundary values and the heights of the rectangles correspond to the class frequencies. The easiest way to draw a histogram is to mark the class mid-point values on the horizontal scale and draw the rectangles symmetrically about the appropriate class mid-point values and touching one another.

231

Problem 11. Table 30.2.

Frequency 2 6 12 14 4 2

Draw a histogram for the data given in

When dealing with unequal class intervals, the histogram must be drawn so that the areas, (and not the heights), of the rectangles are proportional to the frequencies of the classes. The data given

232

Basic Engineering Mathematics

are shown in columns 1 and 2 of Table 30.3. Columns 3 and 4 give the upper and lower class boundaries, respectively. In column 5, the class ranges (i.e. upper class boundary minus lower class boundary values) are listed. The heights of the rectangles frequency are proportional to the ratio , as shown in column 6. class range The histogram is shown in Fig. 30.8.

2 Frequency

3 4 5 Upper Lower Class class class range boundary boundary

6 Height of rectangle

20–40

2

45

15

30

1 2 = 30 15

50–70

6

75

45

30

3 6 = 30 15

80–90

12

95

75

20

9 12 = 20 15

100–110

14

115

95

20

14 10 12 = 20 15

120–140

4

145

115

30

2 4 = 30 15

150–170

2

175

145

30

1 2 = 30 15

Frequency per unit class range

range = 9.1 − 7.1 = 2.0 The size of each class is given approximately by range number of classes

Table 30.3 1 Class

The range of the data is the member having the largest value minus the member having the smallest value. Inspection of the set of data shows that:

Since about seven classes are required, the size of each class is 2.0/7, that is approximately 0.3, and thus the class limits are selected as 7.1 to 7.3, 7.4 to 7.6, 7.7 to 7.9, and so on. 7.35 + 7.05 The class mid-point for the 7.1 to 7.3 class is , i.e. 2 7.65 + 7.35 7.2, for the 7.4 to 7.6 class is , i.e. 7.5, and so on. 2 To assist with accurately determining the number in each class, a tally diagram is produced as shown in Table 30.4. This is obtained by listing the classes in the left-hand column and then inspecting each of the 50 members of the set of data in turn and allocating it to the appropriate class by putting a ‘1’ in the appropriate row. Each fifth ‘1’ allocated to a particular row is marked as an oblique line to help with final counting. Table 30.4 Class 7.1 to 7.3 7.4 to 7.6 7.7 to 7.9 8.0 to 8.2 8.3 to 8.5 8.6 to 8.8 8.9 to 9.1

12/15 10/15 8/15 6/15 4/15 2/15 30

60 85 105 130 Class mid-point values

A frequency distribution for the data is shown in Table 30.5 and lists classes and their corresponding frequencies. Class mid-points are also shown in this table, since they are used when constructing the frequency polygon and histogram.

Problem 12. The masses of 50 ingots in kilograms are measured correct to the nearest 0.1 kg and the results are as shown below. Produce a frequency distribution having about 7 classes for these data and then present the grouped data as (a) a frequency polygon and (b) a histogram. 8.6 7.1 8.4 7.4 8.2

8.2 8.1 7.9 8.8 8.4

7.5 8.3 8.8 8.0 7.7

8.0 8.7 7.2 8.4 8.3

9.1 7.8 8.1 8.5 8.2

8.5 8.7 7.8 8.1 7.9

7.6 8.5 8.2 7.3 8.5

8.2 8.4 7.7 9.0 7.9

111 1111 1111 1111 1111 1111 1111 1 1111 1111 1111 1 11

160

Fig. 30.8

8.0 8.3 7.7 8.1 7.4

Tally

7.8 8.5 7.5 8.6 8.0

Table 30.5 Class 7.1 to 7.3 7.4 to 7.6 7.7 to 7.9 8.0 to 8.2 8.3 to 8.5 8.6 to 8.8 8.9 to 9.1

Class mid-point

Frequency

7.2 7.5 7.8 8.1 8.4 8.7 9.0

3 5 9 14 11 6 2

Presentation of statistical data

A frequency polygon is shown in Fig. 30.9, the co-ordinates corresponding to the class mid-point/frequency values, given in Table 30.5. The co-ordinates are joined by straight lines and the polygon is ‘anchored-down’ at each end by joining to the next class mid-point value and zero frequency.

Frequency

14 12

Frequency polygon

10 8 6 4 2 0

7.2

7.5

7.8 8.1 8.4 8.7 Class mid-point values

9.0

A cumulative frequency distribution is a table giving values of cumulative frequency for the values of upper class boundaries, and is shown in Table 30.6. Columns 1 and 2 show the classes and their frequencies. Column 3 lists the upper class boundary values for the classes given in column 1. Column 4 gives the cumulative frequency values for all frequencies less than the upper class boundary values given in column 3. Thus, for example, for the 7.7 to 7.9 class shown in row 3, the cumulative frequency value is the sum of all frequencies having values of less than 7.95, i.e. 3 + 5 + 9 = 17, and so on. The ogive for the cumulative frequency distribution given in Table 30.6 is shown in Fig. 30.11. The co-ordinates corresponding to each upper class boundary/cumulative frequency value are plotted and the coordinates are joined by straight lines (– not the best curve drawn through the co-ordinates as in experimental work.) The ogive is ‘anchored’ at its start by adding the co-ordinate (7.05, 0). Table 30.6 1 Class

2 Frequency

7.1–7.3 7.4–7.6 7.7–7.9 8.0–8.2 8.3–8.5 8.6–8.8 8.9–9.1

3 5 9 14 11 6 2

8.7

9.0

9.15

8.4

8.85

8.1

8.55

7.8

8.25

7.5

7.95

7.2

7.65

2 0

Histogram

7.35

Frequency

A histogram is shown in Fig. 30.10, the width of a rectangle corresponding to (upper class boundary value – lower class boundary value) and height corresponding to the class frequency. The easiest way to draw a histogram is to mark class mid-point values on the horizontal scale and to draw the rectangles symmetrically about the appropriate class mid-point values and touching one another. A histogram for the data given in Table 30.5 is shown in Fig. 30.10.

Cumulative frequency

Fig. 30.9

14 12 10 8 6 4

233

3 Upper class boundary

4 Cumulative frequency

Less than 7.35 7.65 7.95 8.25 8.55 8.85 9.15

3 8 17 31 42 48 50

50 40 30 20 10

Class mid-point values

Fig. 30.10

7.05 7.35 7.65 7.95 8.25 8.55 8.85 9.15 Upper class boundary values in kilograms

Fig. 30.11 Problem 13. The frequency distribution for the masses in kilograms of 50 ingots is: 7.1 to 7.3 8.0 to 8.2 8.9 to 9.1

3 14 2

7.4 to 7.6 8.3 to 8.5

5 11

7.7 to 7.9 8.6 to 8.8

9 6

Form a cumulative frequency distribution for these data and draw the corresponding ogive.

Now try the following exercise Exercise 111

Further problems on presentation of grouped data (Answers on page 282)

1. The mass in kilograms, correct to the nearest one-tenth of a kilogram, of 60 bars of metal are as shown. Form

234

Basic Engineering Mathematics

a frequency distribution of about 8 classes for these data. 39.8 40.1 40.3 40.0 40.6 39.7 40.0 40.4 39.6 39.3 39.6 40.7 40.2 39.9 40.3 40.2 40.4 39.9 39.8 40.0 40.2 40.1 40.3 39.7 39.9 40.5 39.9 40.5 40.0 39.9 40.1 40.8 40.0 40.0 40.1 40.2 40.1 40.0 40.2 39.9 39.7 39.8 40.4 39.7 39.9 39.5 40.1 40.1 39.9 40.2 39.5 40.6 40.0 40.1 39.8 39.7 39.5 40.2 39.9 40.3 2. Draw a histogram for the frequency distribution given in the solution of Problem 1. 3. The information given below refers to the value of resistance in ohms of a batch of 48 resistors of similar value. Form a frequency distribution for the data, having about 6 classes and draw a frequency polygon and histogram to represent these data diagramatically. 21.0 22.9 23.2 22.1 21.4 22.2

22.4 20.5 22.9 21.8 22.4 21.6

22.8 21.8 21.7 22.0 22.3 21.3

21.5 22.2 21.4 22.7 20.9 22.1

22.6 21.0 22.1 21.7 22.8 21.5

21.1 21.7 22.2 21.9 21.2 22.0

21.6 22.5 22.3 21.1 22.7 23.4

22.3 20.7 21.3 22.6 21.6 21.2

4. The time taken in hours to the failure of 50 specimens of a metal subjected to fatigue failure tests are as shown. Form a frequency distribution, having about 8 classes and unequal class intervals, for these data. 28 21 24 20 27

22 14 22 25 9

23 30 26 23 13

20 23 3 26 35

12 27 21 47 20

24 13 24 21 16

37 23 28 29 20

28 7 40 26 25

21 26 27 22 18

25 19 24 33 22

5. Form a cumulative frequency distribution and hence draw the ogive for the frequency distribution given in the solution to Problem 3. 6. Draw a histogram for the frequency distribution given in the solution to Problem 4. 7. The frequency distribution for a batch of 48 resistors of similar value, measured in ohms, is: 20.5–20.9 22.0–22.4

3 13

21.0–21.4 22.5–22.9

10 9

21.5–21.9 23.0–23.4

11 2

Form a cumulative frequency distribution for these data. 8. Draw an ogive for the data given in the solution of Problem 7. 9. The diameter in millimetres of a reel of wire is measured in 48 places and the results are as shown. 2.10 2.28 2.26 2.16 2.24 2.15 2.11 2.23

2.29 2.18 2.10 2.25 2.05 2.22 2.17 2.07

2.32 2.17 2.21 2.23 2.29 2.14 2.22 2.13

2.21 2.20 2.17 2.11 2.18 2.27 2.19 2.26

2.14 2.23 2.28 2.27 2.24 2.09 2.12 2.16

2.22 2.13 2.15 2.34 2.16 2.21 2.20 2.12

(a) Form a frequency distribution of diameters having about 6 classes. (b) Draw a histogram depicting the data. (c) Form a cumulative frequency distribution. (d) Draw an ogive for the data.

31 Measures of central tendency and dispersion

31.1 Measures of central tendency A single value, which is representative of a set of values, may be used to give an indication of the general size of the members in a set, the word ‘average’ often being used to indicate the single value. The statistical term used for ‘average’ is the arithmetic mean or just the mean. Other measures of central tendency may be used and these include the median and the modal values.

31.2 Mean, median and mode for discrete data

of the values of the members of the set. The median value is obtained by: (a) ranking the set in ascending order of magnitude, and (b) selecting the value of the middle member for sets containing an odd number of members, or finding the value of the mean of the two middle members for sets containing an even number of members. For example, the set: {7, 5, 74, 10} is ranked as {5, 7, 10, 74}, and since it contains an even number of members (four in this case), the mean of 7 and 10 is taken, giving a median value of 8.5. Similarly, the set: {3, 81, 15, 7, 14} is ranked as {3, 7, 14, 15, 81} and the median value is the value of the middle member, i.e. 14. Mode

Mean The arithmetic mean value is found by adding together the values of the members of a set and dividing by the number of members in the set. Thus, the mean of the set of numbers: {4, 5, 6, 9} is: 4+5+6+9 , 4

i.e.

6

In general, the mean of the set: {x1 , x2 , x3 , . . . xn } is x x1 + x2 + x3 + · · · + xn , written as x¯ = n n where is the Greek letter ‘sigma’ and means ‘the sum of’, and x¯ (called x-bar) is used to signify a mean value. Median The median value often gives a better indication of the general size of a set containing extreme values. The set: {7, 5, 74, 10} has a mean value of 24, which is not really representative of any

The modal value, or mode, is the most commonly occurring value in a set. If two values occur with the same frequency, the set is ‘bi-modal’. The set: {5, 6, 8, 2, 5, 4, 6, 5, 3} has a modal value of 5, since the member having a value of 5 occurs three times. Problem 1. Determine the mean, median and mode for the set: {2, 3, 7, 5, 5, 13, 1, 7, 4, 8, 3, 4, 3} The mean value is obtained by adding together the values of the members of the set and dividing by the number of members in the set. Thus, mean value, 2 + 3 + 7 + 5 + 5 + 13 + 1 +7 + 4 + 8 + 3 + 4 + 3 65 = =5 x¯ = 13 13

236

Basic Engineering Mathematics

To obtain the median value the set is ranked, that is, placed in ascending order of magnitude, and since the set contains an odd number of members the value of the middle member is the median value. Ranking the set gives: {1, 2, 3, 3, 3, 4, 4, 5, 5, 7, 7, 8, 13} The middle term is the seventh member, i.e. 4, thus the median value is 4. The modal value is the value of the most commonly occurring member and is 3, which occurs three times, all other members only occurring once or twice. Problem 2. The following set of data refers to the amount of money in £s taken by a news vendor for 6 days. Determine the mean, median and modal values of the set: {27.90, 34.70, 54.40, 18.92, 47.60, 39.68}

by the sum of the frequencies, i.e. mean value x¯ =

where f is the frequency of the class having a mid-point value of x, and so on. Problem 3. The frequency distribution for the value of resistance in ohms of 48 resistors is as shown. Determine the mean value of resistance. 20.5–20.9

3,

21.0–21.4

10,

21.5–21.9

11,

22.0–22.4

13,

22.5–22.9

9,

23.0–23.4

2

The class mid-point/frequency values are: 20.7

Mean value

3, 21.2

10, 21.7

11, 22.2

13, 22.7

9 and 23.2

2

For grouped data, the mean value is given by:

27.90 + 34.70 + 54.40 + 18.92 + 47.60 + 39.68 6 = £37.20 =

The ranked set is: {18.92, 27.90, 34.70, 39.68, 47.60, 54.40} Since the set has an even number of members, the mean of the middle two members is taken to give the median value, i.e. 34.70 + 39.68 = £37.19 2 Since no two members have the same value, this set has no mode. median value =

Now try the following exercise Exercise 112

f1 x1 + f2 x2 + · · · fn xn ( fx) = f1 + f2 + · · · + fn f

Further problems on mean, median and mode for discrete data (Answers on page 282)

In Problems 1 to 4, determine the mean, median and modal values for the sets given. 1. {3, 8, 10, 7, 5, 14, 2, 9, 8} 2. {26, 31, 21, 29, 32, 26, 25, 28} 3. {4.72, 4.71, 4.74, 4.73, 4.72, 4.71, 4.73, 4.72} 4. {73.8, 126.4, 40.7, 141.7, 28.5, 237.4, 157.9}

31.3 Mean, median and mode for grouped data The mean value for a set of grouped data is found by determining the sum of the (frequency × class mid-point values) and dividing

( fx) x¯ = f where f is the class frequency and x is the class mid-point value. Hence mean value, (3 × 20.7) + (10 × 21.2) + (11 × 21.7) + (13 × 22.2) + (9 × 22.7) + (2 × 23.2) x¯ = 48 1052.1 = = 21.919 . . . 48 i.e. the mean value is 21.9 ohms, correct to 3 significant figures.

Histogram The mean, median and modal values for grouped data may be determined from a histogram. In a histogram, frequency values are represented vertically and variable values horizontally. The mean value is given by the value of the variable corresponding to a vertical line drawn through the centroid of the histogram. The median value is obtained by selecting a variable value such that the area of the histogram to the left of a vertical line drawn through the selected variable value is equal to the area of the histogram on the right of the line. The modal value is the variable value obtained by dividing the width of the highest rectangle in the histogram in proportion to the heights of the adjacent rectangles. The method of determining the mean, median and modal values from a histogram is shown in Problem 4. Problem 4. The time taken in minutes to assemble a device is measured 50 times and the results are as shown. Draw a

Measures of central tendency and dispersion

histogram depicting this data and hence determine the mean, median and modal values of the distribution. 14.5–15.5

5,

16.5–17.5

8,

18.5–19.5

16,

20.5–21.5

12,

22.5–23.5

6,

24.5–25.5

3

The histogram is shown in Fig. 31.1. The mean value lies at the centroid of the histogram. With reference to any arbitrary axis, say YY shown at a time of 14 minutes, the position of the horizontal value of the centroid can be obtained from the relationship AM = (am), where A is the area of the histogram, M is the horizontal distance of the centroid from the axis YY , a is the area of a rectangle of the histogram and m is the distance of the centroid of the rectangle from YY . The areas of the individual rectangles are shown circled on the histogram giving a total area of 100 square units. The positions, m, of the centroids of the individual rectangles are 1, 3, 5, . . . units from YY . Thus 100 M = (10 × 1) + (16 × 3) + (32 × 5) + (24 × 7) + (12 × 9) + (6 × 11) 560 M= = 5.6 units from YY 100

i.e.

16

Y

the adjacent rectangles. With reference to Fig. 31.1, this is done by joining AC and BD and drawing a vertical line through the point of intersection of these two lines. This gives the mode of the distribution and is 19.3 minutes.

Now try the following exercise Exercise 113

Further problems on mean, median and mode for grouped data (Answers on page 282)

1. 21 bricks have a mean mass of 24.2 kg, and 29 similar bricks have a mass of 23.6 kg. Determine the mean mass of the 50 bricks. 2. The frequency distribution given below refers to the heights in centimetres of 100 people. Determine the mean value of the distribution, correct to the nearest millimetre. 150–156

5,

157–163

18,

164–170

20

171–177

27,

178–184

22,

185–191

8

3. The gain of 90 similar transistors is measured and the results are as shown.

Mean Median Mode

A

237

B

83.5–85.5

6,

86.5–88.5

39,

92.5–94.5

15,

95.5–97.5

3

89.5–91.5

27,

Frequency

14 5.6

12 10

C

By drawing a histogram of this frequency distribution, determine the mean, median and modal values of the distribution.

24

D

8

32 16

6 4

12

10

2

E

F

6

14 15 16 17 18 19 20 21 22 23 24 25 26 27 Time in minutes Y

Fig. 31.1 Thus the position of the mean with reference to the time scale is 14 + 5.6, i.e. 19.6 minutes. The median is the value of time corresponding to a vertical line dividing the total area of the histogram into two equal parts. The total area is 100 square units, hence the vertical line must be drawn to give 50 units of area on each side. To achieve this with reference to Fig. 31.1, rectangle ABFE must be split so that 50 − (10 + 16) units of area lie on one side and 50 − (24 + 12 + 6) units of area lie on the other. This shows that the area of ABFE is split so that 24 units of area lie to the left of the line and 8 units of area lie to the right, i.e. the vertical line must pass through 19.5 minutes. Thus the median value of the distribution is 19.5 minutes. The mode is obtained by dividing the line AB, which is the height of the highest rectangle, proportionally to the heights of

4. The diameters, in centimetres, of 60 holes bored in engine castings are measured and the results are as shown. Draw a histogram depicting these results and hence determine the mean, median and modal values of the distribution. 2.011–2.014

7,

2.016–2.019

16,

2.021–2.024

23,

2.026–2.029

9,

2.031–2.034

5

31.4 Standard deviation (a) Discrete data The standard deviation of a set of data gives an indication of the amount of dispersion, or the scatter, of members of the set from the measure of central tendency. Its value is the root-mean-square value of the members of the set and for discrete data is obtained as follows: (a) determine the measure of central tendency, usually the mean value, (occasionally the median or modal values are specified),

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Basic Engineering Mathematics

(b) calculate the deviation of each member of the set from the mean, giving (x1 − x¯ ), (x2 − x¯ ), (x3 − x¯ ), . . . , (c) determine the squares of these deviations, i.e. (x1 − x¯ )2 , (x2 − x¯ )2 , (x3 − x¯ )2 , . . . ,

and

(x − x¯ )2 34 = = 5.6˙ n 6

since there are 6 members in the set. Hence, standard deviation, (x − x¯ 2 σ = = 5.6˙ = 2.380 n

(d) find the sum of the squares of the deviations, that is (x1 − x¯ )2 + (x2 − x¯ )2 + (x3 − x¯ )2 , . . . , (e) divide by the number of members in the set, n, giving (x1 − x¯ )2 + (x2 − x¯ )2 + (x3 − x¯ )2 + · · · n (f) determine the square root of (e). The standard deviation is indicated by σ (the Greek letter small ‘sigma’) and is written mathematically as: standard deviation, σ =

(x − x¯ )2 n

where x is a member of the set, x¯ is the mean value of the set and n is the number of members in the set. The value of standard deviation gives an indication of the distance of the members of a set from the mean value. The set: {1, 4, 7, 10, 13} has a mean value of 7 and a standard deviation of about 4.2. The set {5, 6, 7, 8, 9} also has a mean value of 7, but the standard deviation is about 1.4. This shows that the members of the second set are mainly much closer to the mean value than the members of the first set. The method of determining the standard deviation for a set of discrete data is shown in Problem 5.

Problem 5. Determine the standard deviation from the mean of the set of numbers: {5, 6, 8, 4, 10, 3}, correct to 4 significant figures. The arithmetic mean, x¯ =

x

n

5 + 6 + 8 + 4 + 10 + 3 =6 6 (x − x¯ )2 Standard deviation, σ = n =

correct to 4 significant figures.

(b) Grouped data For grouped data, standard deviation

{ f (x − x¯ )2 } f

σ =

where f is the class frequency value, x is the class mid-point value and x¯ is the mean value of the grouped data. The method of determining the standard deviation for a set of grouped data is shown in Problem 6. Problem 6. The frequency distribution for the values of resistance in ohms of 48 resistors is as shown. Calculate the standard deviation from the mean of the resistors, correct to 3 significant figures. 20.5–20.9

3,

21.0–21.4

10,

21.5–21.9

11,

22.0–22.4

13,

22.5–22.9

9,

23.0–23.4

2

The standard deviation for grouped data is given by:

{ f (x − x¯ )2 } f

σ =

From Problem 3, the distribution mean value, x¯ = 21.92, correct to 4 significant figures. The ‘x-values’ are the class mid-point values, i.e. 20.7, 21.2, 21.7, . . .. Thus the (x − x¯ )2 values are (20.7–21.92)2 , (21.2–21.92)2 , (21.7–21.92)2 , . . ., and the f (x − x¯ )2 values are 3(20.7–21.92)2 , 10(21.2–21.92)2 , 11(21.7–21.92)2 , . . .. The f (x − x¯ )2 values are

The (x − x¯ )2 values are: (5 − 6)2 , (6 − 6)2 , (8 − 6)2 , (4 − 6)2 , (10 − 6)2 and (3 − 6)2 .

4.4652 + 5.1840 + 0.5324 + 1.0192

The sum of the (x − x¯ )2 values,

i.e.

(x − x¯ ) = 1 + 0 + 4 + 4 + 16 + 9 = 34 2

+5.4756 + 3.2768 = 19.9532 { f (x − x¯ )2 } 19.9532 = 0.41569 = 48 f

Measures of central tendency and dispersion

and standard deviation, √ {f (x − x¯ )2 } = 0.41569 σ = f = 0.645, correct to 3 significant figures

Now try the following exercise Exercise 114

Further problems on standard deviation (Answers on page 283)

1. Determine the standard deviation from the mean of the set of numbers: {35, 22, 25, 23, 28, 33, 30}

239

4, 7, and 13. These values are signified by Q1 , Q2 and Q3 and called the first, second and third quartile values, respectively. It can be seen that the second quartile value, Q2 , is the value of the middle member and hence is the median value of the set. For grouped data the ogive may be used to determine the quartile values. In this case, points are selected on the vertical cumulative frequency values of the ogive, such that they divide the total value of cumulative frequency into four equal parts. Horizontal lines are drawn from these values to cut the ogive. The values of the variable corresponding to these cutting points on the ogive give the quartile values (see Problem 7). When a set contains a large number of members, the set can be split into ten parts, each containing an equal number of members. These ten parts are then called deciles. For sets containing a very large number of members, the set may be split into one hundred parts, each containing an equal number of members. One of these parts is called a percentile.

correct to 3 significant figures. 2. The values of capacitances, in microfarads, of ten capacitors selected at random from a large batch of similar capacitors are:

Problem 7. The frequency distribution given below refers to the overtime worked by a group of craftsmen during each of 48 working weeks in a year.

34.3, 25.0, 30.4, 34.6, 29.6, 28.7,

25–29

5,

30–34

4,

35–39

7,

33.4, 32.7, 29.0 and 31.3

45–49

12,

50–54

8,

55–59

1

Determine the standard deviation from the mean for these capacitors, correct to 3 significant figures. 3. The tensile strength in megapascals for 15 samples of tin were determined and found to be:

40–44

11,

Draw an ogive for this data and hence determine the quartile values. The cumulative frequency distribution (i.e. upper class boundary/ cumulative frequency values) is:

34.61, 34.57, 34.40, 34.63, 34.63, 34.51, 34.49, 34.61, 34.52, 34.55, 34.58, 34.53, 34.44, 34.48 and 34.40

29.5

5,

34.5

9,

39.5

16,

49.5

39,

54.5

47,

59.5

48

Calculate the mean and standard deviation from the mean for these 15 values, correct to 4 significant figures.

The ogive is formed by plotting these values on a graph, as shown in Fig. 29.2. The total frequency is divided into four equal parts, each having a range of 48/4, i.e. 12. This gives cumulative frequency values of 0 to 12 corresponding to the first quartile, 12 to 24 corresponding to the second quartile, 24 to 36 corresponding to the third quartile and 36 to 48 corresponding to the fourth quartile of the distribution, i.e. the distribution is divided into four equal parts. The quartile values are those of the variable corresponding to cumulative frequency values of 12, 24 and 36, marked Q1 , Q2 and Q3 in Fig. 31.2. These values, correct to the nearest hour, are 37 hours, 43 hours and 48 hours, respectively. The Q2 value is also equal to the median value of the distribution. One measure of the dispersion of a distribution is called the semi-interquartile range and is given by (Q3 − Q1 )/2, and is (48 − 37)/2 in this case, i.e. 5 12 hours.

4. Calculate the standard deviation from the mean for the mass of the 50 bricks given in Problem 1 of Exercise 113, page 237, correct to 3 significant figures. 5. Determine the standard deviation from the mean, correct to 4 significant figures, for the heights of the 100 people given in Problem 2 of Exercise 113, page 237. 6. Calculate the standard deviation from the mean for the data given in Problem 3 of Exercise 113, page 237, correct to 3 decimal places.

31.5 Quartiles, deciles and percentiles Other measures of dispersion which are sometimes used are the quartile, decile and percentile values. The quartile values of a set of discrete data are obtained by selecting the values of members which divide the set into four equal parts. Thus for the set: {2, 3, 4, 5, 5, 7, 9, 11, 13, 14, 17} there are 11 members and the values of the members dividing the set into four equal parts are

44.5

27,

Problem 8. Determine the numbers contained in the (a) 41st to 50th percentile group, and (b) 8th decile group of the set of numbers shown below: 14

22

17

21

30

28

37

7

23

32

24

17

20

22

27

19

26

21

15

29

240

Basic Engineering Mathematics

Now try the following exercise Cumulative frequency

50

Exercise 115

40

Further problems on quartiles, deciles and percentiles (Answers on page 283)

1. The number of working days lost due to accidents for each of 12 one-monthly periods are as shown. Determine the median and first and third quartile values for this data.

30 20

27 37 40 28 23 30 35 24 30 32 31 28 10

25

30

35 Q1 40

Q2 45 Q3 50

55

60

Upper class boundary values, hours

2. The number of faults occurring on a production line in a nine-week period are as shown below. Determine the median and quartile values for the data. 30

27

25

24

27

37

31

27

35

Fig. 31.2 The set is ranked, giving: 7

14

15

17

17

19

20

21

21

22

22

23

24

26

27

28

29

30

32

37

(a) There are 20 numbers in the set, hence the first 10% will be the two numbers 7 and 14, the second 10% will be 15 and 17, and so on. Thus the 41st to 50th percentile group will be the numbers 21 and 22. (b) The first decile group is obtained by splitting the ranked set into 10 equal groups and selecting the first group, i.e. the numbers 7 and 14. The second decile group are the numbers 15 and 17, and so on. Thus the 8th decile group contains the numbers 27 and 28.

3. Determine the quartile values and semi-interquartile range for the frequency distribution given in Problem 2 of Exercise 113, page 236. 4. Determine the numbers contained in the 5th decile group and in the 61st to 70th percentile groups for the set of numbers: 40 32

46 38

28 27

32 33

37 40

42 35

50 25

31 42

48 38

45 41

5. Determine the numbers in the 6th decile group and in the 81st to 90th percentile group for the set of numbers: 43 47 30 25 15 51 17 21 37 33 44 56 40 49 22 36 44 33 17 35 58 51 35 44 40 31 41 55 50 16

32 Probability

32.1 Introduction to probability Probability The probability of something happening is the likelihood or chance of it happening. Values of probability lie between 0 and 1, where 0 represents an absolute impossibility and 1 represents an absolute certainty. The probability of an event happening usually lies somewhere between these two extreme values and is expressed either as a proper or decimal fraction. Examples of probability are: that a length of copper wire has zero resistance at 100◦ C

0

that a fair, six-sided dice will stop with a 3 upwards

1 6

or 0.1667

that a fair coin will land with a head upwards

1 2

or 0.5

that a length of copper wire has some resistance at 100◦ C

1

If p is the probability of an event happening and q is the probability of the same event not happening, then the total probability is p + q and is equal to unity, since it is an absolute certainty that the event either does or does not occur, i.e. p+q=1 Expectation The expectation, E, of an event happening is defined in general terms as the product of the probability p of an event happening and the number of attempts made, n, i.e. E = pn. Thus, since the probability of obtaining a 3 upwards when rolling a fair dice is 16 , the expectation of getting a 3 upwards on four throws of the dice is 16 × 4, i.e. 23 Thus expectation is the average occurrence of an event.

Dependent event A dependent event is one in which the probability of an event happening affects the probability of another ever happening. Let 5 transistors be taken at random from a batch of 100 transistors for test purposes, and the probability of there being a defective transistor, p1 , be determined. At some later time, let another 5 transistors be taken at random from the 95 remaining transistors in the batch and the probability of there being a defective transistor, p2 , be determined. The value of p2 is different from p1 since batch size has effectively altered from 100 to 95, i.e. probability p2 is dependent on probability p1 . Since transistors are drawn, and then another 5 transistors drawn without replacing the first 5, the second random selection is said to be without replacement. Independent event An independent event is one in which the probability of an event happening does not affect the probability of another event happening. If 5 transistors are taken at random from a batch of transistors and the probability of a defective transistor p1 is determined and the process is repeated after the original 5 have been replaced in the batch to give p2 , then p1 is equal to p2 . Since the 5 transistors are replaced between draws, the second selection is said to be with replacement.

32.2 Laws of probability The addition law of probability The addition law of probability is recognised by the word ‘or’ joining the probabilities. If pA is the probability of event A happening and pB is the probability of event B happening, the probability of event A or event B happening is given by pA + pB . Similarly, the probability of events A or B or C or . . . N happening is given by pA + pB + pC + · · · + pN

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Basic Engineering Mathematics

The multiplication law of probability The multiplication law of probability is recognised by the word ‘and’ joining the probabilities. If pA is the probability of event A happening and pB is the probability of event B happening, the probability of eventA and event B happening is given by pA × pB . Similarly, the probability of events A and B and C and . . . N happening is given by pA × pB × pC × · · · × pN

Problem 1. Determine the probabilities of selecting at random (a) a man, and (b) a woman from a crowd containing 20 men and 33 women.

(a) The probability of selecting at random a man, p, is given by the ratio i.e.

p= =

20 20 + 33 20 53

i.e.

(a) the winning horse in a race in which 10 horses are running,

q= =

33 20 + 33 33 53

(a) Since only one of the ten horses can win, the probability of number of winners , selecting at random the winning horse is number of horses 1 or 0.10 i.e. 10 (b) The probability of selecting the winning horse in the first 1 race is . The probability of selecting the winning horse 10 1 . The probability of selecting the in the second race is 10 winning horses in the first and second race is given by the multiplication law of probability, i.e.

probability =

1 1 1 × = 10 10 100

or 0.01

or 0.3774

(b) The probability of selecting at random a women, q, is given by the ratio number of women number in crowd

Problem 3. Calculate the probabilities of selecting at random:

(b) the winning horses in both the first and second races if there are 10 horses in each race.

32.3 Worked problems on probability

number of men number in crowd

Also, 3 attempts are made, hence n = 3 and the expectation, E, is pn, i.e. E = 16 × 3 = 12 or 0.50.

or 0.6226

Problem 4. The probability of a component failing in one 1 , due to excesyear due to excessive temperature is 20 1 1 and due to excessive humidity is . sive vibration is 25 50 Determine the probabilities that during a one-year period a component: (a) fails due to excessive temperature and excessive vibration, (b) fails due to excessive vibration or excessive humidity, and (c) will not fail because of both excessive temperature and excessive humidity.

(Check: the total probability should be equal to 1; p=

20 33 and q = 53 53

thus the total probability, p+q=

20 33 + =1 53 53

hence no obvious error has been made).

Problem 2. Find the expectation of obtaining a 4 upwards with 3 throws of a fair dice.

Expectation is the average occurrence of an event and is defined as the probability times the number of attempts. The probability, p, of obtaining a 4 upwards for one throw of the dice is 16 .

Let pA be the probability of failure due to excessive temperature, then pA =

1 20

and

pA =

19 20

(where pA is the probability of not failing.) Let pB be the probability of failure due to excessive vibration, then pB =

1 25

and

pB =

24 25

Let pC be the probability of failure due to excessive humidity, then pC =

1 50

and

pC =

49 50

Probability

(a) The probability of a component failing due to excessive temperature and excessive vibration is given by: pA × pB =

1 1 1 × = 20 25 500

(b) The probability of a component failing due to excessive vibration or excessive humidity is: p B + pC =

1 3 1 + = 25 50 50

or 0.06

(c) The probability that a component will not fail due excessive temperature and will not fail due to excess humidity is: pA × pC =

931 19 49 × = 20 50 1000

or 0.931

Problem 5. A batch of 100 capacitors contains 73 which are within the required tolerance values, 17 which are below the required tolerance values, and the remainder are above the required tolerance values. Determine the probabilities that when randomly selecting a capacitor and then a second capacitor: (a) both are within the required tolerance values when selecting with replacement, and (b) the first one drawn is below and the second one drawn is above the required tolerance value, when selection is without replacement. (a) The probability of selecting a capacitor within the required 73 . The first capacitor drawn is now tolerance values is 100 replaced and a second one is drawn from the batch of 100. The probability of this capacitor being within the required 73 tolerance values is also . 100 Thus, the probability of selecting a capacitor within the required tolerance values for both the first and the second draw is 73 73 5329 × = 100 100 10 000

or 0.5329

(b) The probability of obtaining a capacitor below the required 17 tolerance values on the first draw is . There are now 100 only 99 capacitors left in the batch, since the first capacitor is not replaced. The probability of drawing a capacitor above the required tolerance values on the second draw is 10 , since there are (100 − 73 − 17), i.e. 10 capacitors above 99 the required tolerance value. Thus, the probability of randomly selecting a capacitor below the required tolerance values and followed by randomly selecting a capacitor above the tolerance values is 17 10 170 17 × = = 100 99 9900 990

Now try the following exercise Exercise 116

or 0.002

243

Further problems on probability (Answers on page 283)

1. In a batch of 45 lamps there are 10 faulty lamps. If one lamp is drawn at random, find the probability of it being (a) faulty and (b) satisfactory. 2. A box of fuses are all of the same shape and size and comprises 23 2 A fuses, 47 5 A fuses and 69 13 A fuses. Determine the probability of selecting at random (a) a 2 A fuse, (b) a 5 A fuse and (c) a 13 A fuse. 3. (a) Find the probability of having a 2 upwards when throwing a fair 6-sided dice. (b) Find the probability of having a 5 upwards when throwing a fair 6-sided dice. (c) Determine the probability of having a 2 and then a 5 on two successive throws of a fair 6-sided dice. 4. The probability of event A happening is 35 and the probability of event B happening is 23 . Calculate the probabilities of (a) both A and B happening, (b) only event A happening, i.e. event A happening and event B not happening, (c) only event B happening, and (d) either A, or B, or A and B happening. 5. When testing 1000 soldered joints, 4 failed during a vibration test and 5 failed due to having a high resistance. Determine the probability of a joint failing due to (a) vibration, (b) high resistance, (c) vibration or high resistance and (d) vibration and high resistance.

32.4 Further worked problems on probability Problem 6. A batch of 40 components contains 5 which are defective. A component is drawn at random from the batch and tested and then a second component is drawn. Determine the probability that neither of the components is defective when drawn (a) with replacement, and (b) without replacement.

(a) With replacement The probability that the component selected on the first draw 7 35 , i.e. . The component is now replaced is satisfactory is 40 8 and a second draw is made. The probability that this component 7 is also satisfactory is . Hence, the probability that both the 8 first component drawn and the second component drawn are satisfactory is:

or 0.0172 7 7 49 × = 8 8 64

or 0.7656

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Basic Engineering Mathematics

(b) Without replacement The probability that the first component drawn is satisfactory 7 is . There are now only 34 satisfactory components left in the 8 batch and the batch number is 39. Hence, the probability of draw34 . Thus ing a satisfactory component on the second draw is 39 the probability that the first component drawn and the second component drawn are satisfactory, i.e. neither is defective, is: 238 7 34 × = 8 39 312

or 0.7628

Problem 7. A batch of 40 components contains 5 which are defective. If a component is drawn at random from the batch and tested and then a second component is drawn at random, calculate the probability of having one defective component, both with and without replacement. The probability of having one defective component can be achieved in two ways. If p is the probability of drawing a defective component and q is the probability of drawing a satisfactory component, then the probability of having one defective component is given by drawing a satisfactory component and then a defective component or by drawing a defective component and then a satisfactory one, i.e. by q × p + p × q With replacement: p=

5 1 = 40 8

and q =

35 7 = 40 8

Hence, probability of having one defective component is:

i.e.

1 7 7 1 × + × 8 8 8 8 7 7 7 = or 0.2188 + 64 64 32

Without replacement: p1 = 18 and q1 = 78 on the first of the two draws. The batch number is now 39 for the second draw, thus, 5 p2 = 39 1 p 1 q2 + q 1 p2 = × 8 70 = 312

35 and q2 = 39 35 7 5 35 + 35 + × = 39 8 39 312 or 0.2244

Problem 8. A box contains 74 brass washers, 86 steel washers and 40 aluminium washers. Three washers are drawn at random from the box without replacement. Determine the probability that all three are steel washers.

Assume, for clarity of explanation, that a washer is drawn at random, then a second, then a third (although this assumption does not affect the results obtained). The total number of washers is 74 + 86 + 40, i.e. 200. The probability of randomly selecting a steel washer on the 86 first draw is . There are now 85 steel washers in a batch of 200 199. The probability of randomly selecting a steel washer on the 85 . There are now 84 steel washers in a batch second draw is 199 of 198. The probability of randomly selecting a steel washer 84 . Hence the probability of selecting a on the third draw is 198 steel washer on the first draw and the second draw and the third draw is: 85 84 614 040 86 × × = = 0.0779 200 199 198 7 880 400

Problem 9. For the box of washers given in Problem 8 above, determine the probability that there are no aluminium washers drawn, when three washers are drawn at random from the box without replacement.

The probability not drawing an aluminium washer on the first of 40 160 draw is 1 − . There are now 199 washers in the , i.e. 200 200 batch of which 159 are not aluminium washers. Hence, the probability of not drawing an aluminium washer on the second draw 159 is . Similarly, the probability of not drawing an aluminium 199 158 . Hence the probability of not washer on the third draw is 198 drawing an aluminium washer on the first and second and third draws is 4 019 520 160 159 158 × × = = 0.5101 200 199 198 7 880 400

Problem 10. For the box of washers in Problem 8 above, find the probability that there are two brass washers and either a steel or an aluminium washer when three are drawn at random, without replacement.

Two brass washers (A) and one steel washer (B) can be obtained in any of the following ways:

1st draw

2nd draw

3rd draw

A A B

A B A

B A A

Probability

Two brass washers and one aluminium washer (C) can also be obtained in any of the following ways: 1st draw

2nd draw

3rd draw

A A C

A C A

C A A

Thus there are six possible ways of achieving the combinations specified. If A represents a brass washer, B a steel washer and C an aluminium washer, then the combinations and their probabilities are as shown:

First

DRAW Second

Third

PROBABILITY

A

A

B

A

B

A

B

A

A

A

A

C

A

C

A

C

A

A

73 86 74 × × = 0.0590 200 199 198 86 73 74 × × = 0.0590 200 199 198 74 73 86 × × = 0.0590 200 199 198 73 40 74 × × = 0.0274 200 199 198 40 73 74 × × = 0.0274 200 199 198 74 73 40 × × = 0.0274 200 199 198

The probability of having the first combination or the second, or the third, and so on, is given by the sum of the probabilities, i.e. by 3 × 0.0590 + 3 × 0.0274, that is, 0.2592 Now try the following exercise Exercise 117

Further problems on probability (Answers on page 283)

1. The probability that component A will operate satisfactorily for 5 years is 0.8 and that B will operate satisfactorily over that same period of time is 0.75. Find the probabilities that in a 5 year period: (a) both components operate satisfactorily, (b) only component A will operate satisfactorily, and (c) only component B will operate satisfactorily.

245

2. In a particular street, 80% of the houses have telephones. If two houses selected at random are visited, calculate the probabilities that (a) they both have a telephone and (b) one has a telephone but the other does not have telephone. 3. Veroboard pins are packed in packets of 20 by a machine. In a thousand packets, 40 have less than 20 pins. Find the probability that if 2 packets are chosen at random, one will contain less than 20 pins and the other will contain 20 pins or more. 4. A batch of 1 kW fire elements contains 16 which are within a power tolerance and 4 which are not. If 3 elements are selected at random from the batch, calculate the probabilities that (a) all three are within the power tolerance and (b) two are within but one is not within the power tolerance. 5. An amplifier is made up of three transistors, A, B and C. 1 , The probabilities of A, B or C being defective are 20 1 1 and , respectively. Calculate the percentage of 25 50 amplifiers produced (a) which work satisfactorily and (b) which have just one defective transistor. 6. A box contains 14 40 W lamps, 28 60 W lamps and 58 25 W lamps, all the lamps being of the same shape and size. Three lamps are drawn at random from the box, first one, then a second, then a third. Determine the probabilities of: (a) getting one 25 W, one 40 W and one 60 W lamp, with replacement, (b) getting one 25 W, one 40 W and one 60 W lamp without replacement, and (c) getting either one 25 W and two 40 W or one 60 W and two 40 W lamps with replacement.

246

Basic Engineering Mathematics

4. Determine the mean, median and modal values for the lengths given in metres:

Assignment 14

28, 20, 44, 30, 32, 30, 28, 34, 26, 28 This assignment covers the material in Chapters 30 to 32. The marks for each question are shown in brackets at the end of each question. 1. A company produces five products in the following proportions: Product A

24

Product B

6

Product D

9

Product E

18

Product C

15

(3)

5. The length in millimetres of 100 bolts is as shown below: 50–56 71–77

6 30

57–63 78–84

16 19

64–70 85–91

22 7

Determine for the sample (a) the mean value, and (b) the standard deviation, correct to 4 significant figures. (9) 6. The number of faulty components in a factory in a 12 week period is: 14 12 16 15 10 13 15 11 16 19 17 19

Draw (a) a horizontal bar chart, and (b) a pie diagram to represent these data visually. (7) 2. State whether the data obtained on the following topics are likely to be discrete or continuous: (a) the number of books in a library (b) the speed of a car (c) the time to failure of a light bulb

(3)

3. Draw a histogram, frequency polygon and ogive for the data given below which refers to the diameter of 50 components produced by a machine. Class intervals

Frequency

1.30–1.32 mm 1.33–1.35 mm 1.36–1.38 mm 1.39–1.41 mm 1.42–1.44 mm 1.45–1.47 mm 1.48–1.50 mm

4 7 10 12 8 5 4 (10)

Determine the median and the first and third quartile values. (3) 7. Determine the probability of winning a prize in a lottery by buying 10 tickets when there are 10 prizes and a total of 5000 tickets sold. (2) 8. A sample of 50 resistors contains 44 which are within the required tolerance value, 4 which are below and the remainder which are above. Determine the probability of selecting from the sample a resistor which is (a) below the required tolerance, and (b) above the required tolerance. Now two resistors are selected at random from the sample. Determine the probability, correct to 3 decimal places, that neither resistor is defective when drawn (c) with replacement, and (d) without replacement. (e) If a resistor is drawn at random from the batch and tested, and then a second resistor is drawn from those left, calculate the probability of having one defective component when selection is without replacement. (13)

33 Introduction to differentiation

33.1 Introduction to calculus Calculus is a branch of mathematics involving or leading to calculations dealing with continuously varying functions. Calculus is a subject that falls into two parts:

Given that f (x) = 5x2 + x − 7 determine:

Problem 2. (i) (iii)

f (2) ÷ f (1)

(ii)

f (3 + a)

f (3 + a) − f (3)

(iv)

f (3 + a) − f (3) a

(i) differential calculus (or differentiation) and (ii) integral calculus (or integration). Differentiation is used in calculations involving rates of change (see section 33.10), velocity and acceleration, and maximum and minimum values of curves (see ‘Engineering Mathematics’).

f (x) = 5x2 + x − 7 (i) f (2) = 5(2)2 + 2 − 7 = 15 f (1) = 5(1)2 + 1 − 7 = −1

33.2 Functional notation In an equation such as y = 3x2 + 2x − 5, y is said to be a function of x and may be written as y = f (x). An equation written in the form f (x) = 3x2 + 2x − 5 is termed functional notation. The value of f (x) when x = 0 is denoted by f (0), and the value of f (x) when x = 2 is denoted by f (2) and so on. Thus when

15 = −15 −1 (ii) f (3 + a) = 5(3 + a)2 + (3 + a) − 7 f (2) ÷ f (1) =

= 5(9 + 6a + a2 ) + (3 + a) − 7 = 45 + 30a + 5a2 + 3 + a − 7 = 41 + 31a + 5a2 (iii) f (3) = 5(3)2 + 3 − 7 = 41 f (3 + a) − f (3) = (41 + 31a + 5a2 ) − (41) = 31a + 5a2

f (x) = 3x2 + 2x − 5, then f (0) = 3(0)2 + 2(0) − 5 = −5

and

f (2) = 3(2)2 + 2(2) − 5 = 11 and so on.

Problem 1. If f (x) = 4x2 − 3x + 2 find: f (0), f (3), f (−1) and f (3) − f (−1) f (x) = 4x2 − 3x + 2 f (0) = 4(0)2 − 3(0) + 2 = 2 f (3) = 4(3)2 − 3(3) + 2 = 36 − 9 + 2 = 29 f (−1) = 4(−1)2 − 3(−1) + 2 = 4 + 3 + 2 = 9 f (3) − f (−1) = 29 − 9 = 20

(iv)

f (3 + a) − f (3) 31a + 5a2 = = 31 + 5a a a

Now try the following exercise Exercise 118

Further problems on functional notation (Answers on page 283)

1. If f (x) = 6x2 − 2x + 1 find f (0), f (1), f (2), f (−1) and f (−3) 2. If f (x) = 2x2 + 5x − 7 f (2) − f (−1)

find

f (1),

f (2),

f (−1),

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Basic Engineering Mathematics

3. Given f (x) = 3x3 + 2x2 − 3x + 2 prove that 1 f (1) = f (2) 7 4. If f (x) = −x2 + 3x + 6 find f (2), f (2 + a), f (2 + a) − f (2) and

f (2 + a) − f (2) a

f (x ) 10

B

f (x ) ⫽ x 2

8 6

33.3 The gradient of a curve

4

(a) If a tangent is drawn at a point P on a curve, then the gradient of this tangent is said to be the gradient of the curve at P. In Figure 33.1, the gradient of the curve at P is equal to the gradient of the tangent PQ.

2

C

A

0

1

D

1.5

2

3

x

Fig. 33.3

f (x ) Q

(ii) the gradient of chord AC =

P

4−1 f (2) − f (1) = =3 2−1 1

(iii) the gradient of chord AD 0

x

=

Fig. 33.1 (b) For the curve shown in Figure 33.2, let the points A and B have co-ordinates (x1 , y1 ) and (x2 , y2 ), respectively. In functional notation, y1 = f (x1 ) and y2 = f (x2 ) as shown. f (x ) B

f (1.5) − f (1) 2.25 − 1 = = 2.5 1.5 − 1 0.5

(iv) if E is the point on the curve (1.1, f (1.1)) then the gradient of chord AE f (1.1) − f (1) 1.1 − 1 1.21 − 1 = = 2.1 0.1 =

(v) if F is the point on the curve (1.01, f (1.01)) then the gradient of chord AF A

C

f (x2)

f (x1) E 0

x1

D x2

x

Fig. 33.2 The gradient of the chord AB BD − CD BC = AC ED f (x2 ) − f (x1 ) = (x2 − x1 )

f (1.01) − f (1) 1.01 − 1 1.0201 − 1 = = 2.01 0.01

=

Thus as point B moves closer and closer to point A the gradient of the chord approaches nearer and nearer to the value 2. This is called the limiting value of the gradient of the chord AB and when B coincides with A the chord becomes the tangent to the curve.

=

(c) For the curve f (x) = x2 shown in Figure 33.3: (i) the gradient of chord AB =

9−1 f (3) − f (1) = =4 3−1 2

Now try the following exercise Exercise 119 A further problem on the gradient of a curve (Answers on page 283) 1. Plot the curve f (x) = 4x2 − 1 for values of x from x = −1 to x = +4. Label the co-ordinates (3, f (3)) and (1, f (1)) as J and K, respectively. Join points J and K to form

Introduction to differentiation

the chord JK. Determine the gradient of chord JK. By moving J nearer and nearer to K determine the gradient of the tangent of the curve at K.

33.4 Differentiation from first principles (i) In Figure 33.4, A and B are two points very close together on a curve, δx (delta x) and δy (delta y) representing small increments in the x and y directions, respectively. y

249

Summarising, the differential coefficient, dy f (x + δx) − f (x) δy = f (x) = limit = limit δx→0 δx δx→0 dx δx Problem 3. Differentiate from first principles f (x) = x2 and determine the value of the gradient of the curve at x = 2 To ‘differentiate from first principles’ means ‘to find f (x)’ by using the expression f (x + δx) − f (x) f (x) = limit δx→0 δx f (x) = x2

B(x ⫹ dx, y ⫹ dy ) dy A (x, y )

f (x ⫹dx ) dx

f (x )

x

0

As δx → 0, {2x + δx} → {2x + 0}. Thus f (x) = 2x, i.e. the differential coefficient of x 2 is 2x. At x = 2, the gradient of the curve, f (x) = 2(2) = 4

Fig. 33.4 Gradient of chord AB =

δy δx

however,

δy = f (x + δx) − f (x)

Hence

δy f (x + δx) − f (x) = δx δx

δy approaches a limiting value and As δx approaches zero, δx the gradient of the chord approaches the gradient of the tangent at A. (ii) When determining the gradient of a tangent to a curve there are two notations used. The gradient of the curve at A in Figure 33.4 can either be written as: f (x + δx) − f (x) δy limit or limit δx→0 δx δx→0 δx dy δy = limit dx δx→0 δx In functional notation, f (x+δx) − f (x) f (x)= limit δx→0 δx In Leibniz notation,

(iii)

Substituting (x + δx) for x gives f (x + δx) = (x + δx)2 = x2 + 2xδx + δx2 , hence 2 (x + 2xδx + δx2 ) − (x2 )

f (x) = limit δx→0 δx 2 2xδx + δx = limit = limit {2x + δx} δx→0 δx→0 δx

dy is the same as f (x) and is called the differential dx coefficient or the derivative. The process of finding the differential coefficient is called differentiation.

Problem 4.

By definition,

Find the differential coefficient of y = 5x f (x + δx) − f (x) dy = f (x) = limit δx → 0 δx dx

The function being differentiated is y = f (x) = 5x. Substituting (x + δx) for x gives: f (x + δx) = 5(x + δx) = 5x + 5δx. Hence (5x + 5δx) − (5x) dy

= f (x) = limit δx→0 dx δx 5δx = limit = limit{5} δx→0 δx→0 δx Since the term δx does not appear in {5} the limiting value as δx → 0 of {5} is 5. dy = 5, i.e. the differential coefficient of 5x is 5. dx The equation y = 5x represents a straight line of gradient 5 (see Chapter 12).

Thus

dy or f (x)) means ‘the gradient dx of the curve’, and since the gradient of the line y = 5x is 5 this result can be obtained by inspection. Hence, in general, if y = kx dy (where k is a constant), then the gradient of the line is k and dx or f (x) = k.

The ‘differential coefficient’(i.e.

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Basic Engineering Mathematics

Problem 5.

Find the derivative of y = 8

y = f (x) = 8. Since there are no x-values in the original equation, substituting (x + δx) for x still gives f (x + δx) = 8. Hence f (x + δx) − f (x) dy

= f (x) = limit δx→0 dx δx

8−8 = limit δx→0 δx

= limit

δx → 0

=0

8xδx + 4δx2 + 5δx δx

dy = 0. dx The equation y = 8 represents a straight horizontal line and the gradient of a horizontal line is zero, hence the result could have been determined by inspection. ‘Finding the derivative’ means ‘finding the gradient’, hence, in general, for any horizontal line dy if y = k (where k is a constant) then = 0. dx Differentiate from first principles f (x) = 2x3

f (x + δx) = 2(x + δx) = 2(x + δx)(x + 2xδx + δx ) 2

2

= 2(x3 + 3x2 δx + 3xδx2 + δx3 ) = 2x + 6x δx + 6xδx + 2δx 2

2

3

dy f (x + δx) − f (x) = f (x) = limit δx→0 dx δx 3 2 (2x + 6x δx + 6xδx2 + 2δx3 ) − (2x3 ) = limit δx → 0 δx 2 6x δx + 6xδx2 + 2δx3 = limit δx → 0 δx ! 2 = limit 6x + 6xδx + 2δx2 δx → 0

Hence f (x) = 6x 2 , i.e. the differential coefficient of 2x 3 is 6x 2 .

Problem 7. Find the differential coefficient of y = 4x2 + 5x − 3 and determine the gradient of the curve at x = −3

y = f (x) = 4x + 5x − 3 2

f (x + δx) = 4(x + δx)2 + 5(x + δx) − 3 = 4(x + 2xδx + δx ) + 5x + 5δx − 3 2

= limit {8x + 4δx + 5} δx → 0

Now try the following exercise Exercise 120

Further problems on differentiation from first principles (Answers on page 283)

In Problems 1 to 12, differentiate from first principles. 2. y = 7x

3. y = 4x

3

  

dy = f (x) = 8x + 5 dx At x = −3, the gradient of the curve dy = = f (x) = 8(−3) + 5 = −19 dx

1. y = x Substituting (x + δx) for x gives

3

   

i.e.

Thus, when y = 8,

Problem 6.

dy f (x + δx) − f (x)

= f (x) = limit δx → 0 δx dx  2 (4x + 8xδx + 4δx2 + 5x + 5δx − 3)    −(4x2 + 5x − 3) = limit δx → 0  δx  

2

= 4x2 + 8xδx + 4δx2 + 5x + 5δx − 3

4. y = 5x3

2

5. y = −2x2 + 3x − 12

6. y = 23

7. f (x) = 9x

8. f (x) =

9. f (x) = 9x2

10. f (x) = −7x3

11. f (x) = x2 + 15x − 4 13. Determine 14. Find

2x 3

12. f (x) = 4

d (4x3 − 1) from first principles dx

d (3x2 + 5) from first principles dx

33.5 Differentiation of y = ax n by the general rule From differentiation by first principles, a general rule for differentiating axn emerges where a and n are any constants. This rule is: if or,

if

y = ax n then f (x) = ax n

then

dy = anx n−1 dx f (x) = anx n−1

(Each of the results obtained in worked problems 3 and 7 may be deduced by using this general rule). When differentiating, results can be expressed in a number of ways.

Introduction to differentiation

dy = 6x, dx 2 (ii) if f (x) = 3x then f (x) = 6x,

For example: (i) if y = 3x2 then

Problem 10.

251

1 If f (t) = 5t + √ find f (t) t3

(iii) the differential coefficient of 3x2 is 6x, (iv) the derivative of 3x2 is 6x, and (v)

d (3x2 ) = 6x dx

Problem 8. Using the general rule, differentiate the following with respect to x: (a) y = 5x7

√ (b) y = 3 x

(c) y =

3 1 f (t) = 5t + √ = 5t + 13 = 5t 1 + t − 2 3 t t2 3 − 3 −1 3 5 t 2 = 5t 0 − t − 2 Hence f (t) = (5)(1)t 1−1 + − 2 2

i.e.

f (t) = 5 −

4 x2 Problem 11.

(a) Comparing y = 5x7 with y = axn shows that a = 5 and n = 7. Using the general rule, dy = anx n−1 = (5)(7)x7−1 = 35x 6 dx 1 √ 1 (b) y = 3 x = 3x 2 . Hence a = 3 and n = 2

dy 1 1 = anxn−1 = (3) x 2 −1 dx 2 3 −1 3 3 = x 2 = = √ 1 2 2 x 2x 2 4 (c) y = 2 = 4x−2 . Hence a = 4 and n = −2 x dy = anxn−1 = (4)(−2)x−2−1 dx 8 = −8x−3 = − 3 x Problem 9. Find the differential coefficient of √ 2 4 y = x3 − 3 + 4 x5 + 7 5 x

i.e.

√ 2 4 y = x3 − 3 + 4 x5 + 7 5 x 2 3 y = x − 4x−3 + 4x5/2 + 7 from the laws of 5 indices (see Chapter 3) 2 dy = (3)x3−1 − (4)(−3)x−3−1 dx 5

5 (5/2)−1 + (4) +0 x 2

6 = x2 + 12x−4 + 10x3/2 5 √ dy 6 2 12 = x + 4 + 10 x 3 i.e. dx 5 x

y=

5 2t 2

3 =5− √ 2 t5

Differentiate y =

(since t 0 = 1)

(x + 2)2 with respect to x x

x2 + 4x + 4 x2 4x 4 (x + 2)2 = = + + x x x x x

y = x1 + 4 + 4x−1

i.e. Hence

3

dy = 1x1−1 + 0 + (4)(−1)x−1−1 = x0 − 4x−2 dx 4 = 1 − 2 (since x0 = 1) x

Now try the following exercise Exercise 121

Further problems on differentiation of y = ax n by the general rule (Answers on page 283)

In Problems 1 to 8, determine the differential coefficients with respect to the variable. √ 1. y = 7x4 2. y = x 3. y =

√ t3

4. y = 6 +

1 x3

5 1 − √ +2 x2 x7

1 1 5. y = 3x − √ + x x

6. y =

7. y = 3(t − 2)2

8. y = (x + 1)3

9. Using the general rule for axn check the results of Problems 1 to 12 of Exercise 120, page 250. 10. Differentiate f (x) = 6x2 − 3x + 5 and find the gradient of the curve at (a) x = −1, and (b) x = 2. 11. Find the differential coefficient of y = 2x3 + 3x2 − 4x − 1 and determine the gradient. of the curve at x = 2. 12. Determine the derivative of y = −2x3 + 4x + 7 and determine the gradient of the curve at x = −1.5.

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Basic Engineering Mathematics

33.6 Differentiation of sine and cosine functions

y y ⫽ cos u

⫹

Figure 33.5(a) shows a graph of y = sin θ. The gradient is continually changing as the curve moves from 0 to A to B to C to dy D. The gradient, given by , may be plotted in a corresponding dθ position below y = sin θ, as shown in Figure 33.5(b).

(a)

π 2

0 ⫺

π

3π 2

2π u radians

3π 2

2π u radians

y A ⫹ (a)

0

dy ⫹ du (b) 0 ⫺

y ⫽ sin u

π 2

π

B

⫺

D 3π 2

2π u radians

d (cos u) ⫽ ⫺sin u du

C dy ⫹ dx (b) ⫺

D′

d (sin u) ⫽ cos u du A′ 0

π

π 2

C′ 3π 2

2π

Fig. 33.6 If a similar exercise is followed for y = cos θ then the graphs dy of Figure 33.6 result, showing to be a graph of sin θ, but dθ displaced by π radians. If each point on the curve y = sin θ (as shown in Figure 33.5(a)) π π 3π were to be made negative, (i.e. + is made − , − is made 2 2 2 3π + , and so on) then the graph shown in Figure 33.6(b) would 2 result. This latter graph therefore represents the curve of −sin θ.

u radians

B′

Fig. 33.5 (i) At 0, the gradient is positive and is at its steepest. Hence 0 is a maximum positive value. (ii) Between 0 and A the gradient is positive but is decreasing in value until at A the gradient is zero, shown as A .

dy = −sin θ dθ It may also be shown that:

Thus,

(iv) If the gradient of y = sin θ is further investigated between dy is seen B and C and C and D then the resulting graph of dθ to be a cosine wave. Hence the rate of change of sin θ is cos θ, i.e. then

if y = cos θ,

if y = cos aθ,

(iii) Between A and B the gradient is negative but is increasing in value until at B the gradient is at its steepest. Hence B is a maximum negative value.

if y = sin θ

π

π 2

and

dy = −a sin(aθ + α) dθ (where a and α are constants)

if y = cos(aθ + α),

Problem 12. the variable:

dy = cos θ dθ

dy = −a sin aθ dθ (where a is a constant)

(3)

(4)

Differentiate the following with respect to

(a) y = 2 sin 5θ

(b) f (t) = 3 cos 2t

It may also be shown that: if y = sin aθ,

and

(a) y = 2 sin 5θ

dy = a cos aθ dθ (where a is a constant)

(1)

dy = (a cos aθ + α) dθ

(2)

if y = sin (aθ + α),

(where a and α are constants)

dy = (2)(5 cos 5θ) = 10 cos 5θ dθ

from equation (1)

(b) f (t) = 3 cos 2t f (t) = (3)(−2 sin 2t) = −6 sin 2t

from equation (3)

Introduction to differentiation

253

Now try the following exercise Problem 13. Find the y = 7 sin 2x − 3 cos 4x

differential

coefficient

of Exercise 122

y = 7 sin 2x − 3 cos 4x

1. Differentiate with respect to x:

dy = (7)(2 cos 2x) − (3)(−4 sin 4x) from dx equations (1) and (3) = 14 cos 2x + 12 sin 4x Problem 14. the variable:

Further problems on the differentiation of sine and cosine functions (Answers on page 283)

(a) y = 4 sin 3x

(b) y = 2 cos 6x

2. Given f (θ) = 2 sin 3θ − 5 cos 2θ, find f (θ)

Differentiate the following with respect to

(a) f (θ) = 5 sin(100π θ − 0.40) (b) f (t) = 2 cos(5t + 0.20) (a) If f (θ) = 5 sin (100π θ − 0.40) f (θ) = 5[100π cos (100π θ − 0.40)] from equation (2),

3. An alternating current is given by i = 5 sin 100t amperes, where t is the time in seconds. Determine the rate of di change of current (i.e. ) when t = 0.01 seconds. dt 4. v = 50 sin 40t volts represents an alternating voltage, v, where t is the time in seconds. At a time of 20 × 10−3 secdv ). onds, find the rate of change of voltage (i.e. dt 5. If f (t) = 3 sin (4t + 0.12) − 2 cos (3t − 0.72) determine f (t).

where a = 100π = 500πcos(100π θ − 0.40)

33.7 Differentiation of eax and ln ax

(b) If f (t) = 2 cos (5t + 0.20) f (t) = 2[−5 sin (5t + 0.20)] from equation (4), where a = 5 = −10 sin(5t + 0.20) Problem 15. An alternating voltage is given by: v = 100 sin 200t volts, where t is the time in seconds. Calculate the rate of change of voltage when (a) t = 0.005 s and (b) t = 0.01 s v = 100 sin 200t volts. The rate of change of v is given by

dv . dt

dv = (100)(200 cos 200t) = 20 000 cos 200t dt (a) When t = 0.005 s, dv = 20 000 cos(200)(0.005) = 20 000 cos 1 dt cos 1 means ‘the cosine of 1 radian’ (make sure your calculator is on radians – not degrees). dv = 10 806 volts per second Hence dt (b) When t = 0.01 s, dv = 20 000 cos(200)(0.01) = 20 000 cos 2 dt dv = −8323 volts per second Hence dt

A graph of y = ex is shown in Figure 33.7(a). The gradient of the dy and is continually changing. curve at any point is given by dx By drawing tangents to the curve at many points on the curve dy and measuring the gradient of the tangents, values of for dx corresponding values of x may be obtained. These values are dy against x shown graphically in Figure 33.7(b). The graph of dx x is identical to the original graph of y = e . It follows that: if y = ex ,

then

dy = ex dx

It may also be shown that if y = eax ,

then

dy = aeax dx

dy = (2)(6e6x ) = 12e6x dx A graph of y = ln x is shown in Figure 33.8(a). The gradient of the dy and is continually changing. curve at any point is given by dx By drawing tangents to the curve at many points on the curve dy for and measuring the gradient of the tangents, values of dx corresponding values of x may be obtained. These values are dy against x shown graphically in Figure 33.8(b). The graph of dx dy 1 is the graph of = . It follows that: dx x Therefore if y = 2e6x , then

if y = ln x,

then

dy 1 = dx x

254

Basic Engineering Mathematics

y

Thus if y = ln 4x, then

20 15

⫺3

⫺2

10

Problem 16. the variable:

5

(a) y = 3e2x

⫺1

dy dx

y ⫽ ex

1

0 (a)

2

3

x

20 15

dy 1 = dx x

Differentiate the following with respect to (b) f (t) =

4 3e5t

dy = (3)(2e2x ) = 6e2x dx 4 4 (b) If f (t) = 5t = e−5t , then 3e 3 4 20 20 −5t f (t) = (−5e ) = − e−5t = − 5t 3 3 3e (a) If y = 3e2x then

dy ⫽ ex dx

10

Differentiate y = 5 ln 3x

Problem 17. 5 ⫺3

⫺2

⫺1

0 (b)

1

2

3

x

Fig. 33.7 2

Exercise 123 y ⫽ ln x

1 1

2

3

4

5

6

x

⫺1 ⫺2

Further problems on the differentiation of eax and ln ax (Answers on page 283)

1. Differentiate with respect to x: 2 (a) y = 5e3x (b) y = 2x 7e 2. Given f (θ) = 5 ln 2θ − 4 ln 3θ, determine f (θ) 3. If f (t) = 4 ln t + 2, evaluate f (t) when t = 0.25 5 dy when x = 1, given y = 3e4x − 3x + 4. Evaluate dx 2e 8 ln 5x. Give the answer correct to 3 significant figures.

(a) dy dx

1 dy 5 = (5) = dx x x

Now try the following exercise

y

0

If y = 5 ln 3x, then

2 dy 1 ⫽ dx x

1.5 1.0

33.8 Summary of standard derivatives

0.5

The five main differential coefficients used in this chapter are summarised in Table 33.1.

0

1

2

3

4

5

6

x

Table 33.1

(b)

Fig. 33.8

y or f (x)

It may also be shown that

axn sin ax cos ax eax

if y = ln ax,

then

dy 1 = dx x

(Note that in the latter expression, the constant ‘a’does not appear dy in the term). dx

ln ax

dy or f (x) dx anxn−1 a cos ax −a sin ax aeax 1 x

Introduction to differentiation

Problem 18. Find the gradient y = 3x2 − 7x + 2 at the point (1, −2)

of

the

dy = 6x − 7 dx At the point (1, −2), x = 1, hence gradient = 6(1) − 7 = −1

Problem 19. mine

If y =

dy dx

3 2 − 2 sin 4x + x + ln 5x, deterx2 e

2 3 − 2 sin 4x + x + ln 5x x2 e = 3x−2 − 2 sin 4x + 2e−x + ln 5x 1 dy = 3(−2x−3 ) − 2(4 cos 4x) + 2 −e−x + dx x 6 2 1 = − 3 − 8 cos 4x − x + x e x y=

Now try the following exercise Exercise 124

Further problems on standard derivatives (Answers on page 283)

1. Find the gradient of the curve y = 2x4 + 3x3 − x + 4 at the points (a) (0, 4) and (b) (1, 8) 2. Differentiate: y=

2 2 + 2 ln 2x − 2(cos 5x + 3 sin 2x) − 3x x2 e

33.9 Successive differentiation When a function y = f (x) is differentiated with respect to x the dy or f (x). If the expresdifferential coefficient is written as dx sion is differentiated again, the second differential coefficient is d2 y obtained and is written as 2 (pronounced dee two y by dee x dx squared) or f

(x) ( pronounced f double-dash x). By successive differentiation further higher derivatives such as d3 y d4 y and 4 may be obtained. 3 dx dx dy d2 y d3 y d4 y = 20x3 , 2 = 60x2 , 3 = 120x, 4 = 120 Thus if y = 5x4 , dx dx dx dx d5 y and 5 = 0 dx Problem 20.

If f (x) = 4x5 − 2x3 + x − 3, find f

(x)

f (x) = 4x5 − 2x3 + x − 3

curve

If y = 3x2 − 7x + 2, then gradient =

255

f (x) = 20x4 − 6x2 + 1 f (x) = 80x 3 − 12x

Problem 21. d2 y dx2

4x(20x 2 − 3)

or

1 √ 2 4 − x, determine Given y = x3 − 2 + 2x 3 x

2 4 1 √ − x y = x3 − 2 + 3 x 2x 1 1 2 = x3 − 4x−2 + x−1 − x 2 3 2 1 1 2 2 1 dy = 3x − 4(−2x−3 ) + −1x−2 − x− 2 dx 3 2 2 1 dy 1 1 = 2x2 + 8x−3 − x−2 − x− 2 i.e. dx 2 2 1 d2 y −4 = 4x + (8)(−3x ) − −2x−3 dx2 2 1 1 3 − − x− 2 2 2 1 −3 −4 −3 = 4x − 24x + 1x + x 2 4 d2 y 24 1 1 i.e. = 4x − + + √ dx 2 x4 x3 4 x3

Now try the following exercise Exercise 125

Further problems on successive differentiation (Answers on page 283)

d2 y d3 y (b) 3 2 dx dx 2 2 1 3 √ 2. (a) Given f (t) = t − 3 + − t + 1, determine 5 t t f

(t) 1. If y = 3x4 + 2x3 − 3x + 2, find (a)

(b) Evaluate f

(t) in part (a) when t = 1 3. If y = 3 sin 2t + cos t, find

d2 y dt 2

4. If f (θ) = 2 ln 4θ, show that f

(θ) = −

2 θ2

33.10 Rates of change If a quantity y depends on and varies with a quantity x then the dy . Thus, for example, rate of change of y with respect to x is dx dp the rate of change of pressure p with height h is . dh A rate of change with respect to time is usually just called ‘the rate of change’, the ‘with respect to time’ being assumed. Thus,

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Basic Engineering Mathematics

for example, a rate of change of current, i, is dθ change of temperature, θ, is , and so on. dt

di and a rate of dt

Problem 22. The length L metres of a certain metal rod at temperature t ◦ C is given by: L = 1 + 0.00003t + 0.0000004t 2 . Determine the rate of change of length, in mm/◦ C, when the temperature is (a) 100◦ C and (b) 250◦ C

Problem 25. The pressure p of the atmosphere at height h above ground level is given by p = p0 e−h/c , where p0 is the pressure at ground level and c is a constant. Determine the rate of change of pressure with height when p0 = 105 Pascals and c = 6.2 × 104 at 1550 metres.

The rate of change of pressure with height is

dp . dh

Since p = p0 e−h/c then

dL The rate of change of length means dt

1 p0 dp = (p0 ) − e−h/c = − e−h/c dh c c

Since length L = 1 + 0.00003t + 0.0000004t , then dl = 0.00003 + 0.0000008t dt dL = 0.00003 + (0.0000008)(100) (a) When t = 100◦ C, dt = 0.00011m/◦ C = 0.11 mm/◦ C dL = 0.00003 + (0.0000008)(250) (b) When t = 250◦ C, dt = 0.00023m/◦ C = 0.23 mm/◦ C 2

When p0 = 105 , c = 6.2 × 104 and h = 1550, then rate of change of pressure, 105 dp 4 =− e−(1550/6.2×10 ) dh 6.2 × 104 =−

10 −0.025 e 6.2

= −1.573 Pa/m Problem 23. The luminous intensity I candelas of a lamp at varying voltage V is given by: I = 5 × 10−4 V2 . Determine the voltage at which the light is increasing at a rate of 0.4 candelas per volt.

The rate of change of light with respect to voltage is given by

dI . dV

Since I = 5 × 10−4 V2 , dI = (5 × 10−4 )(2 V) = 10 × 10−4 V = 10−3 V dV When the light is increasing at 0.4 candelas per volt then +0.4 = 10−3 V, from which, voltage 0.4 V = −3 = 0.4 × 10+3 = 400 volts 10 Problem 24. Newton’s law of cooling is given by: θ = θ0 e−kt , where the excess of temperature at zero time is θ0◦ C and at time t seconds is θ ◦ C. Determine the rate of change of temperature after 50 s, given that θ0 = 15◦ C and k = −0.02 The rate of change of temperature is

dθ . dt

dθ = (θ0 )(−ke−kt ) = −kθ0 e−kt dt When θ0 = 15, k = −0.02 and t = 50, then dθ = −(−0.02)(15)e−(−0.02)(50) = 0.30e1 = 0.815◦ C/s dt Since θ = θ0 e−kt then

Now try the following exercise Exercise 126

Further problems on rates of change (Answers on page 284)

1. An alternating current, i amperes, is given by i = 10 sin 2πft, where f is the frequency in hertz and t the time in seconds. Determine the rate of change of current when t = 12 ms, given that f = 50 Hz. 2. The luminous intensity, I candelas, of a lamp is given by I = 8 × 10−4 V2 , where V is the voltage. Find (a) the rate of change of luminous intensity with voltage when V = 100 volts, and (b) the voltage at which the light is increasing at a rate of 0.5 candelas per volt. 3. The voltage across the plates of a capacitor at any time t seconds is given by v = V e−t/CR , where V , C and R are constants. Given V = 200 volts, C = 0.10 × 10−6 farads and R = 2 × 106 ohms, find (a) the initial rate of change of voltage, and (b) the rate of change of voltage after 0.2 s. 4. The pressure p of the atmosphere at height h above ground level is given by p = p0 e−h/c , where p0 is the pressure at ground level and c is a constant. Determine the rate of change of pressure with height when p0 = 1.013 × 105 Pascals and c = 6.05 × 104 at 1450 metres.

34 Introduction to integration

34.1 The process of integration The process of integration reverses the process of differentiation. In differentiation, if f (x) = 2x2 then f (x) = 4x. Thus, the integral of 4x is 2x2 , i.e. integration is the process of moving from f (x) to f (x). By similar reasoning, the integral of 2t is t 2 . Integration is a process of summation or adding parts together ) and an elongated S, shown as , is) used to replace)the words ‘the integral of ’. Hence, from above, 4x = 2x2 and 2t is t 2 . dy indicates that In differentiation, the differential coefficient dx a function of x is being differentiated with respect to x, the dx indicating that it is ‘with respect to x’. In integration the variable of integration is shown by adding d (the variable) after the function to be integrated. ) Thus 4x dx means ‘the integral of 4x with respect to x’, and ) 2t dt means ‘the integral of 2t with respect to t’ As the differential coefficient of 2x2 is 4x, hence ) stated above, 4x dx = 2x2 . However, the differential coefficient of 2x2 + 7 is ) also 4x. Hence 4x dx could also be equal to 2x2 + 7. To allow for the possible presence of a constant, whenever the process of integration is performed, a constant ‘c’ is added to the result. ) ) Thus 4x dx = 2x2 + c and 2t dt = t 2 + c ‘c’ is called the arbitrary constant of integration.

34.2 The general solution of integrals of the form axn The general solution of integrals of the form and n are constants and n = −1, is given by:

)

axn dx, where a

Using this rule gives: * 3 3x4+1 + c = x5 + c (i) 3x4 dx = 4+1 5 * 4 3 4 t 3+1 t dt dx = +c (ii) 9 9 3+1 4 1 4 t + c = t4 + c = 9 4 9 Both of these results may be checked by differentiation.

34.3 Standard integrals d (sin ax) = a cos ax dx Since integration is the reverse process of differentiation it follows that: * a cos ax dx = sin ax + c * 1 or cos ax dx = sin ax + c a By similar reasoning * 1 sin ax dx = − cos ax + c a * 1 ax ax e dx = e + c a * 1 dx = ln x + c and x

From Chapter 33,

ax n dx =

ax n+1 +c n+1

)

axn+1 + c except when n = −1 axn dx = n+1 ) −1 ) 1 dx = ln x + c When n = −1, then x dx = x A list of standard integrals is summarised in Table 34.1. From above,

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Basic Engineering Mathematics

)

Table 34.1 Standard integrals * (i)

axn dx

=

cos ax dx

=

sin ax dx

=

eax dx

=

1 dx x

=

axn+1 +c n+1 (except when n = −1)

* (ii) (iii) (iv)

* * *

(v)

1 sin ax + c a 1 − cos ax + c a 1 ax e +c a ln x + c

* Problem 1.

* 3x2 dx

Determine: (a)

i.e. each term is integrated separately. (This splitting up of terms only applies for addition and subtraction). * 5 Hence 2 + x − 6x2 dx 7 1+1 5 x x2+1 − (6) +c = 2x + 7 1+1 2+1 2 5 x x3 = 2x + − (6) + c 7 2 3 = 2x +

* The general rule is

2t 3 dt

(b)

5 2 2 + x − 6x dx may be written as: 7 * * * 5 x dx − 6x2 dx 2 dx + 7

axn+1 +c n+1

axn dx =

Note that when an integral contains more than one term there is no need to have an arbitrary constant for each; just a single constant at the end is sufficient.

(a) When a = 3 and n = 2 then *

3x2+1 + c = x3 + c 2+1

3x2 dx =

5 2 x − 2x 3 + c 14

* Problem 4.

Determine:

(b) When a = 2 and n = 3 then * 2t 3 dt =

2t 4 1 2t 3+1 +c= + c = t4 + c 3+1 4 2

Each of these results may be checked by differentiating them. * Problem 2.

(a)

Determine (a)

(b)

) 8 dx is the same as 8x0 dx and, using the general rule when a = 8 and n = 0 gives: * 8x0+1 8x0 dx = + c = 8x + c 0+1 ) In general, if k is a constant then k dx = kx + c

3x−2+1 +c −2 + 1 3x−1 −3 = + c = −3x−1 + c = +c −1 x

3x−2 dx =

* Problem 5.

* 2x1 dx =

*

1

1

3x 2 dx =

* Problem 3.

Determine:

√ 3 x dx

* Problem 6.

3

3x 2 3x 2 +1 +c= 3 +c 1 +1 2 2 √ 3 3 2 = (3) x 2 = 2x 2 + c = 2 x 3 +c 3

√ 3 x dx =

2x2 2x1+1 +c = +c 1+1 2 = x2 + c

5 2 + x − 6x2 dx 7

Determine:

m √ For fractional powers it is necessary to appreciate that n am = a n , √ 1 from which, x = x 2 . Hence,

*

(b) When a = 2 and n = 1, then 2x dx =

*

2x dx

)

*

) ) ) 3 dx = 3x−2 dx. Using the standard integral, axn dx when x2 a = 3 and n = −2 gives:

* 8 dx

3 dx x2

Determine:

5 √ dx x

Introduction to integration

*

*

1

1

5x 2 5x− 2 +1 +c= +c 1 1 − +1 2 2 1 √ 2 = (5) x 2 + c = 10 x + c 1

5 √ dx = x

1

5x− 2 dx =

Problem 7. Determine: * * 3 x − 2x dx (b) (1 − x)2 dx (a) 3x

(a) Rearranging into standard integral form gives: * 3 * 3 2x x − 2x x − dx = dx 3x 3x 3x * 2 2 x − dx = 3 3 2+1 2 1 x − x+c = 3 2+1 3 3 2 1 x − x+c = 3 3 3 1 3 2 = x − x+c 9 3 (b) Rearranging

)

(1 − x)2 dx gives:

* (1 − 2x + x2 ) dx = x −

x2+1 2x1+1 + +c 1+1 2+1

x3 2x2 + +c = x− 2 3 1 = x − x2 + x3 + c 3 This problem shows that) functions often have to be rearranged into the standard form of axn dx before it is possible to integrate them. Problem 8. Determine: * * (a) 5 cos 3x dx (b) 3 sin 2x dx

259

(b) From Table 34.1(iii), * * 1 3 sin 2x dx = 3 sin 2x dx = (3) − cos 2x + c 2 3 = − cos 2x + c 2 * Problem 9.

Determine: (a)

* 5e3x dx

(b)

6 dx e2x

(a) From Table 34.1(iv), * * 1 3x 5e3x dx = 5 e3x dx = (5) e +c 3 5 3x e +c 3

= * (b)

6 dx = e2x

*

6e−2x dx = 6

= (6) =−

*

e−2x dx

1 −2x e + c = −3e−2x + c −2

3 +c e2x

Problem 10. Determine: * 2 * 3x − 1 3 dx (b) dx (a) 5x x * * 3 1 1 3 dx = dx 5 x 5 x 3 = ln x + c (from Table 34.1(v)) 5 * 2 * 2 1 3x 3x − 1 − dx = dx x x x * 3x2 1 + ln x + c dx = = 3x − x 2 3 = x 2 + ln x + c 2

* (a)

(b)

3 dx = 5x

Now try the following exercise (a) From Table 34.1(ii), * * 5 cos 3x dx = 5 cos 3x dx 1 sin 3x + c = (5) 3 5 = sin 3x + c 3

Exercise 127

Further problems on standard integrals (Answers on page 284)

Determine the following integrals: * * 1. (a) 4 dx (b) 7x dx * * 2 2 5 3 x dx x dx 2. (a) (b) 5 6

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Basic Engineering Mathematics

*

* (3 + 2x − 4x2 ) dx

3. (a)

* 4. (a) *

3x2 − 5x dx x

3 dx 4x4 * 2 (b) √ dx x * (b) 7 sin 3x dx

(b)

* 3 cos 2x dx * * 9. (a)

(2 + x)2 dx *

4 dx 3x2 * √ 6. (a) x dx

8. (a)

* (b)

* (b)

5. (a)

7. (a)

(x + 5x2 ) dx

(b) 3

* dx 2 3 e5x * 2 x −1 (b) dx x

3 2x e dx 4

(b)

2 dx 3x

3 x3 (4 − x2 ) dx = 4x − 3 −2 −2 (3)3 (−2)3 = 4(3) − − 4(−2) − 3 3 −8 = {12 − 9} − −8 − 3 1 1 or 8.33 = {3} − −5 =8 3 3 3

Problem 12. Evaluate: * * 2 x(3 + 2x) dx (b) (a)

−1

0

*

2

(a)

*

=

1

3 3 1 3 +c − +c = 3 3 1 1 = (9 + c) − +c 3 2 =8 3

x2 dx =

x3 +c 3

−2

1

* (a) 1

2

3x dx =

3x2 2

2 = 1

1 1 =6−1 = 4 2 2

3 2 (2) 2 or

−

4.5

3 2 (1) 2

2 0

− {0 + 0}

−1

1 x4 5x2 − +x 4 2 −1 (−1)4 5(−1)2 1 5 − +1 − − + (−1) = 4 2 4 2 1 5 1 5 − +1 − − −1 =2 = 4 2 4 2

=

* Problem 13.

π/2

3 sin 2x dx

Evaluate: 0

*

π/2 0

Problem 11. Evaluate: * 3 * 2 3x dx (b) (4 − x2 ) dx (a)

3(2)2 2(2)3 + 2 3

3x2 2x3 + 2 3

1 16 = 11 or 11.33 3 3 * 1 4 * 1 4 5x2 x x − 5x2 + x x − + (b) dx = dx x x x x −1 −1 * 1 = (x3 − 5x + 1) dx

3

Note that the ‘c’ term always cancels out when limits are applied and it need not be shown with definite integrals.

(3x + 2x2 ) dx =

=6+

Applying the limits gives: 3

x4 − 5x2 + x dx x

0

34.4 Definite integrals

*

2

x(3 + 2x) dx =

0

Integrals containing an arbitrary constant c in their results are called indefinite integrals since their precise value cannot be determined without further information. Definite integrals are those in which limits are applied. If an expression is written as [x]ba ‘b’ is called the upper limit and ‘a’ the lower limit. The operation of applying the limits is defined as: [x]ba = (b) − (a) The increase in the of the integral x2 as x increases from ) 3 value 2 1 to 3 is written as 1 x dx

1

π/2 1 3 sin 2x dx = (3) − cos 2x 2 0 π/2 3 3 π/2 = − cos 2x = − [cos 2x]0 2 2 0 , π 3 + cos 2 − {cos 2(0)} =− 2 2 3 = − [(cos π) − (cos 0)] 2 3 3 = − [( − 1) − (1)] = − (−2) = 3 2 2

Introduction to integration

*

* Problem 14.

Evaluate:

4 cos 3t dt

(b)

0

8. (a) 2

2 1 4 sin 3t 4 cos 3t dt = (4) = [sin 3t]21 3 3 1 1 4 = [sin 6 − sin 3] 3 Note that limits of trigonometric functions are always expressed in radians – thus, for example, sin 6 means the sine of 6 radians = −0.279415.. * 2 4 4 cos 3t dt = [(−0.279415..) − (0.141120..)] Hence 3 1 4 = [−0.420535] = −0.5607 3

3

2

2 dx 2x 3e −1 * 3 2 2x + 1 (b) dx x 1

3e3t dt *

1

*

*

1

7. (a)

2

261

2 dx 3x

2

*

*

2

4e2x dx

Problem 15. Evaluate: (a)

(b)

1

1

each correct to 4 significant figures. *

2

(a) 1

*

4

(b) 1

4

4 2x 2 e = 2[e2x ]21 = 2[e4 − e2 ] 2 1 = 2[54.5982 − 7.3891] = 94.42 4 3 3 3 du = ln x = [ ln 4 − ln 1] 4x 4 4 1 3 = [1.3863 − 0] = 1.040 4

Further problems on definite integrals (Answers on page 284)

*

* x(1 + 4x) dx (b)

1 π

3 cos θ dθ 2

4. (a) 0

*

2 sin 2x dx *

5 cos 3x dx 0

2

−1 π/2

(b)

x3 + 2x2 − 3x dx x 4 cos x dx

*

0 2

(b)

π/6 1

6. (a)

1

*

π/3

5. (a)

y ⫽ f (x)

Evaluate the following definite integrals (where necessary, correct to 4 significant figures): * 1 * 4 3 2 t dt 5x dx (b) 1. (a) 1 −1 4 * 2 * 3 2. (a) (3 − x2 ) dx (b) (x2 − 4x + 3) dx

*

y

0

4e2x dx =

Exercise 128

3. (a)

The area shown shaded in Figure 34.1 may be determined using approximate methods such as the trapezoidal rule, the mid-ordinate rule or Simpson’s rule (see Chapter 25) or, more precisely, by using integration.

3 du, 4x

Now try the following exercise

−1 2

34.5 Area under a curve

x⫽a

x⫽b

Fig. 34.1 The shaded area in Figure 34.1 is given by: * shaded area =

b

*

b

y dx =

f (x) dx

a

a

Thus, determining the area under a curve by integration merely involves evaluating a definite integral, as shown in Section 34.4. There are several instances in engineering and science where the area beneath a curve needs to be accurately determined. For example, the areas between limits of a: velocity/time graph gives distance travelled, force/distance graph gives work done, voltage/current graph gives power, and so on. Should a curve ) drop below the x-axis, then y (= f (x)) becomes negative and f (x) dx is negative. When determining such areas by integration, a negative sign is placed before the integral. For the curve shown in Figure 34.2, the total shaded area is given by (area E + area F + area G). By integration, * total shaded area =

b

* f (x) dx −

a

c

f (x) dx b

*

d

+

3 sin t dt *

x

0

f (x) dx c

π/2

(b) π/4

(3 sin 2x − 2 cos 3x) dx

(Note that this is not the same as

)d a

f (x) dx).

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Basic Engineering Mathematics

y

Area of trapezium = y ⫽ f (x)

0

between parallel sides) 1 = (5 + 11)(3) = 24 square units] 2

G

E a

b

c

F

d

1 (sum of parallel sides) 2 (perpendicular distance

x

Problem 17. The velocity v of a body t seconds after a certain instant is: (2t 2 + 5)m/s. Find by integration how far it moves in the interval from t = 0 to t = 4 s.

Fig. 34.2

It it usually necessary to sketch a curve in order to check whether it crosses the x-axis.

Since 2t 2 + 5 is a quadratic expression, the curve v = 2t 2 + 5 is a parabola cutting the v-axis at v = 5, as shown in Figure 34.4. v (m/s)

Problem 16. Determine the area enclosed by y = 2x + 3, the x-axis and ordinates x = 1 and x = 4.

40 v ⫽ 2t 2 ⫹ 5

y = 2x + 3 is a straight line graph as shown in Figure 34.3, where the required area is shown shaded.

30

y 20

y ⫽ 2x ⫹ 3

12 10

10 8 5 6 0

4 2

0

1

2

3

t (s)

4

Fig. 34.4 1

2

3

4

5

x

The distance travelled is given by the area under the v/t curve (shown shaded in Figure 34.4). *

Fig. 34.3 *

y dx

=

1 4

=

*

(2x + 3) dx

=

1

4 2x2 = + 3x 2 1 = [(16 + 12) − (1 + 3)] = 24 square units [This answer may be checked since the shaded area is a trapezium.

i.e.

4

v dt =

0

4

By integration, shaded area = *

4

By integration, shaded area =

2t 3 + 5t 3

4

(2t 2 + 5) dt

0

0

2(43 ) + 5(4) − (0) 3

distance travelled = 62.67 m

Problem 18. Sketch the graph y = x3 + 2x2 − 5x − 6 between x = −3 and x = 2 and determine the area enclosed by the curve and the x-axis.

Introduction to integration

A table of values is produced and the graph sketched as shown in Figure 34.5 where the area enclosed by the curve and the x-axis is shown shaded. x

−3

−2

−1

0

1

2

x3

−27

−8

−1

0

1

8

2x2

18

8

2

0

2

8

−5x

15

10

5

0

−5

−10

−6

−6

−6

−6

−6

−6

−6

y

0

4

0

−6

−8

0

263

1 1 2 1 = 3 − −2 − −12 − 3 12 4 3 12 1 3 = 5 − −15 3 4 1 = 21 or 21.08 square units 12 Problem 19. Determine the area enclosed by the curve y = 3x2 + 4, the x-axis and ordinates x = 1 and x = 4 by (a) the trapezoidal rule,

(b) the mid-ordinate rule,

(c) Simpson’s rule, and

(d) integration.

The curve y = 3x2 + 4 is shown plotted in Figure 34.6.

y

x 0 1.0 1.5 2.0 2.5 3.0 3.5 4.0 y 4 7 10.75 16 22.75 31 40.75 52

y

y ⫽ 3x 2 ⫹ 4

y ⫽ x 3 ⫹ 2x 2 ⫺ 5x ⫺ 6

50

40 ⫺3

⫺2

⫺1

0

1

2

x

30

20 6

10 4 0

1

2

3

4

x

Fig. 34.5 ) −1 )2 Shaded area = −3 y dx − −1 y dx, the minus sign before the second integral being necessary since the enclosed area is below the x-axis. Hence,

Fig. 34.6 The trapezoidal rule, the mid-ordinate rule and Simpson’s rule are discussed in Chapter 25, page 191. (a) By the trapezoidal rule,

shaded area * −1 = (x3 + 2x2 − 5x − 6) dx −3

* −

2 −1

(x3 + 2x2 − 5x − 6) dx

−1 4 2 4 x 2x3 5x2 2x3 5x2 x + − − 6x − + − − 6x = 4 3 2 4 3 2 −3 −1 81 1 2 5 45 − − +6 − − 18 − + 18 = 4 3 2 4 2 1 2 5 16 − 10 − 12 − − − +6 − 4+ 3 4 3 2

area = (width of interval)

1 first + last + sum of 2 ordinate remaining ordinates

Selecting 6 intervals each of width 0.5 gives: 1 area = (0.5) (7 + 52) + 10.75 + 16 2

+ 22.75 + 31 + 40.75

= 75.375 square units

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Basic Engineering Mathematics

(b) By the mid-ordinate rule, area = (width of interval)(sum of mid-ordinates).

*

Selecting 6 intervals, each of width 0.5 gives the midordinates as shown by the broken lines in Figure 34.6.

width of interval

+4

  first + last ordinates

sum of even ordinates

= 



Exercise 129

(d) By integration, shaded area =

*

4

y dx =

1

(3x + 4) dx = [x + 2

3

1

4x]41

= (64 + 16) − (1 + 4) = 75 square units Integration gives the precise value for the area under a curve. In this case Simpson’s rule is seen to be the most accurate of the three approximate methods. Problem 20. Find the area enclosed by the curve π y = sin 2x, the x-axis and the ordinates x = 0 and x = 3 A sketch of y = sin 2x is shown in Figure 34.7.

0

Fig. 34.7

2. Show by integration that the area of the triangle formed by the line y = 2x, the ordinates x = 0 and x = 4 and the x-axis is 16 square units. 3. Sketch the curve y = 3x2 + 1 between x = −2 and x = 4. Determine by integration the area enclosed by the curve, the x-axis and ordinates x = −1 and x = 3. Use an approximate method to find the area and compare your result with that obtained by integration. 4. The force F newtons acting on a body at a distance x metres from a)fixed point is given by: F = 3x + 2x2 . If x work done = x12 F dx, determine the work done when the body moves from the position where x1 = 1 m to that when x2 = 3 m.

5. y = 5x; y ⫽ sin 2x

π/3

π/2

Further problems on area under curves (Answers on page 284)

In Problems 5 to 9, sketch graphs of the given equations, and then find the area enclosed between the curves, the horizontal axis and the given ordinates.

y 1

or 0.75 square units

1. Show by integration that the area of a rectangle formed by the line y = 4, the ordinates x = 1 and x = 6 and the x-axis is 20 square units.

= 75 square units

4

3 1 1 + = 4 2 4

Now try the following exercise

1 (0.5)[(7 + 52) + 4(10.75 + 22.75 3 +40.75) + 2(16 + 31)]

*

sin 2x dx 0

sum of + 2  remaining odd  ordinates

Selecting 6 intervals, each of width 0.5 gives: area =

π/3

π/3 1 = − cos 2x 2 0 2π 1 1 = − cos − − cos 0 2 3 2 1 1 1 = − − − − (1) 2 2 2

+ 35.7 + 46.2)

* y dx =

0

= 74.85 square units

1 area = 3

π/3

Shaded area =

Thus, area = (0.5)(8.7 + 13.2 + 19.2 + 26.7

(c) By Simpson’s rule,

2π , i.e. π radians). 2

(Note that y = sin 2x has a period of

x = 1, x = 4

6. y = 2x − x + 1;

x = −1, x = 2 π 7. y = 2 sin 2x; x = 0, x = 4 π 8. y = 5 cos 3t; t = 0, t = 6 9. y = (x − 1)(x − 3); x = 0, x = 3 2

π

x

10. The velocity v of a vehicle t seconds after a certain instant is given by: v = (3t 2 + 4) m/s. Determine how far it moves in the interval from t = 1 s to t = 5 s.

Introduction to integration

Assignment 15 This assignment covers the material contained in Chapters 33 and 34. The marks for each question are shown in brackets at the end of each question 1. Differentiate from first principles f (x) = 2x2 + x − 1 (5) √ dy (4) 2. If y = 4x5 − 2 x determine dx 1 3 (4) 3. Given f (x) = 2 − find f (x) x x 4. Differentiate the following with respect to the variable: (a) y = 4 sin 2x − 5 cos 3x (b) y = 2e3t − 3 ln 7t

5. Find the gradient of the curve y = 5x + 2x − 3 at the point (1, 4) (4) 6. Given y = 3x2 −

7. Newtons law of cooling is given by: θ = θ0 e−kt , where the excess of temperature at zero time is θ0◦ C and at time t seconds is θ ◦ C. Determine the rate of change of temperature after 40 s, correct to 3 decimal places, given that θ0 = 16◦ C and k = −0.01 (4) 8. Determine the following: * (a) (2 − 3x + 5x2 )dx * √ (b) (4 cos 2x − 2 x)dx

2 √ d y + x find 2 x3 dx 2

(6)

(4)

9. Evaluate the following definite integrals, correct to 4 significant figures: * 2 1 dx 2x2 − (a) x 1 * π/3 3 sin 2xdx (b) *

(4) 2

265

0 1

(c) 0

1 3e2x − √ dx x

(10)

10. Sketch the curve y = 2x2 + 5 between x = −1 and x = 3. Determine, by integration, the area enclosed by the curve, the x-axis and ordinates x = −1 and x = 3. (5)

List of formulae

Laws of indices: am × an = am+n am/n =

√ n

Exponential series: am = am−n an a−n =

am

(am )n = amn

1 an

ex = 1 + x +

x2 x3 + + ··· 2! 3!

(valid for all values of x)

a0 = 1

Theorem of Pythagoras: b2 = a2 + c 2

Quadratic formula: If ax + bx + c = 0 2

then

x=

−b ±

√ b2 − 4ac 2a

A b

c

Equation of a straight line: B

y = mx + c

Definition of a logarithm: If y = ax

then

x = loga y

a

C

Areas of plane figures: (i) Rectangle

Area = l × b

Laws of logarithms: log (A × B) = log A + log B log

A = log A − log B B

b

log An = n × log A l

List of formulae

(ii) Parallelogram

Area = b × h

For a sector of circle: arc length,

s=

shaded area = h

θ◦ (2πr) = rθ 360

θ◦ 1 (πr 2 ) = r 2 θ 360 2

(θ in rad) (θ in rad)

Equation of a circle, centre at origin, radius r: x2 + y2 = r 2

b

Equation of a circle, centre at (a, b), radius r: (iii) Trapezium

Area =

(x − a)2 + ( y − b)2 = r 2

1 (a + b)h 2

a

Volumes and surface areas of regular solids: h

(i) Rectangular prism (or cuboid) Volume = l × b × h

b

Surface area = 2(bh + hl + lb)

Area = 12 × b × h

(iv) Triangle

l

h h

b

(ii) Cylinder b

Volume = πr 2 h Total surface area = 2πrh + 2πr 2

(v) Circle

Area = πr 2

Circumference = 2π r r h s

r

u r

(iii) Pyramid If area of base = A and perpendicular height = h then : Radian measure:

2π radians = 360 degrees

Volume =

1 3

×A×h

267

268

List of formulae

Mid-ordinate rule Area ≈ (width of interval)(sum of mid-ordinates)

h

Total surface area = sum of areas of triangles forming sides + area of base

Simpson’s rule 1 width of first + last Area ≈ ordinate 3 interval sum of remaining sum of even +2 +4 odd ordinates ordinates

(iv) Cone Volume = 13 πr 2 h

Mean or average value of a waveform:

Curved surface area = πrl Total surface area = π rl + πr

l

2

area under curve length of base sum of mid-ordinates = number of mid-ordinates

mean value, y =

h

Triangle formulae:

r

b c a = = sin A sin B sin C

Since rule: (v) Sphere Cosine rule:

Volume = 43 πr 3

a2 = b2 + c2 − 2bc cos A

Surface area = 4πr 2 A c

B r

Area of any triangle = 1 ab sin C 2

= =

Areas of irregular figures by approximate methods: Trapezoidal rule 1 first + last width of Area ≈ interval 2 ordinate

+ sum of remaining ordinates

b C

a

1 × base × perpendicular height 2 or

1 ac sin B 2

[s(s − a)(s − b)(s − c)]

or

1 bc sin A 2

where s =

a+b+c 2

For a general sinusoidal function y = A sin(ωt ± α), then A = amplitude ω 2π ω ω 2π α

= angular velocity = 2πf rad/s = periodic time T seconds = frequency, f hertz = angle of lead or lag (compared with y = A sin ωt)

List of formulae

Cartesian and polar co-ordinates:

Grouped data: fx mean, x¯ = f

If co-ordinate (x, y) = (r, θ) then r=

x2 + y2

and θ = tan−1

y x

-0 !1 . . f (x − x¯ )2 / standard deviation, σ = f

If co-ordinate (r, θ) = (x, y) then x = r cos θ

and y = r sin θ

Arithmetic progression:

Standard derivatives

If a = first term and d = common difference, then the arithmetic progression is: a, a + d, a + 2d, . . .

y or f(x)

dy or f (x) dx

axn

anxn−1

The n’th term is: a + (n − 1)d n Sum of n terms, S = [2a + (n − 1)d] 2

sin ax

a cos ax

cos ax

−a sin ax

Geometric progression:

eax

If a = first term and r = common ratio, then the geometric progression is: a, ar, ar 2 , . . .

ln ax

aeax 1 x

The n’th term is: ar n−1 Sum of n terms, Sn =

a(1 − r n ) (1 − r)

If −1 < r < 1, S∞ =

a (1 − r)

or

a(r n − 1) (r − 1)

Standard integrals y axn

Statistics:

cos ax

Discrete data: mean, x¯ =

sin ax x

n

standard deviation, σ =

eax

(x − x¯ )2 n

1 x

y dx

xn+1 + c (except when n = − 1) n+1 1 sin ax + c a 1 − cos ax + c a 1 ax e +c a a

ln x + c

269

Answers to exercises

Exercise 1 (Page 2) 1. 19

2. −66

3. 565

4. −2136

5. −36 121

6. 107 701

7. 16

8. 1487

9. −225

5. (a)

5 12

(b)

3 49

6. (a)

3 5

(b) 11

7. (a)

1 13

(b)

5 12

8. (a)

8 15

(b)

9. −

1 9

10. 1

1 6

11. 5

13 126

13. 2

28 55

14. 400 litre

12 23

4 5

10. 5914

11. 189

12. −70 872

13. −15 333

14. (a) 1827

(b) 4158

15. (a) 8613

(b) 1752

16. (a) 1026

(b) 2233

15. (a) £60

17. (a) 10 304

(b) −4433

18. (a) 48

(b) 89

Exercise 5 (Page 9)

19. (a) 259

(b) 56

20. (a) 456

(b) 284

1. 91 mm to 221 mm

2. 81 cm to 189 cm to 351 cm

3. £1.71 and £3.23

4. 17 g

5. 72 kg : 27 kg

6. 5

21. (a) 474

10 100 (b) 593 79 187

12. −

22. 18 kg

24. d = 64 mm, A = 136 mm, B = 10 mm

23. 89.25 cm

(b) £36, £16

7. (a) 2 h 10 min

(b) 4 h 20 min

Exercise 6 (Page 11)

Exercise 2 (Page 4) 1. (a) 2

(b) 210

2. (a) 3

(b) 180

1. 11.989

2. −31.265

3. (a) 5

(b) 210

4. (a) 15

(b) 6300

4. 10.906

5. 2.244 6

5. (a) 14

(b) 420 420

6. (a) 14

(b) 53 900

6. (a) 24.81

(b) 24.812

7. (a) 0.006 39

(b) 0.006 4

Exercise 3 (Page 5) 1. 59

2. 14

3. 88

4. 5

5. −107

6. 57

Exercise 4 (Page 8) 1. (a)

9 10

3. (a) 8

51 52

(b)

3 16

(b) 1

9 40

2. (a)

43 77

4. (a) 1

16 21

7. 68

13 20 41 9. (a) 1 50 8. (a)

21 25 11 (b) 4 40 (b)

1 80 1 (c) 14 8 (c)

3. 24.066

141 500 7 (d) 15 20 (d)

3 125 17 (e) 16 80

(e)

47 63

10. 0.444 44

11. 0.629 63

12. 1.563

(b)

13. 53.455

14. 13.84

15. 8.69

(b)

17 60

16. 12.52 mm

17. 2400

Answers to exercises

Exercise 7 (Page 12)

271

Exercise 11 (Page 19) (b) 8.731 × 10−1

1. (a) 5.7%

(b) 37.4%

(c) 128.5%

1. (a) 1.351 × 103

2. (a) 21.2%

(b) 79.2%

(c) 169%

2. (a) 1.723 1 × 103

3. (a) 496.4 t

(b) 8.657 g

(c) 20.73 s

3. (a) 1.35 × 102

4. 2.25%

5. (a) 14%

6. 37.8 g

7. 7.2%

(b) 15.67%

(c) 5.36%

8. A 0.6 kg, B 0.9 kg, C 0.5 kg

9. 54%, 31%, 15%, 0.3 t 11. 13.5 mm, 11.5 mm

4. (a) 2 × 10−2

(c) 5.734 × 10−2

(b) 3.129 × 10−3

(c) 1.119 9 × 103

(b) 1.1 × 10−5 (b) 1.5 × 10−3

5. (a) 2.71 × 103 kg m−3

(b) 4.4 × 10−1 (d) 5.11 × 10−1 MeV

10. 20 000 kg

(c) 3.767 3 × 10

12. 600 kW

(e) 9.578 97 × 107 C kg−1

2

(f ) 2.241 × 10−2 m3 mol−1

Exercise 12 (Page 20)

Exercise 8 (Page 15)

1. (a) 100 × 103 W = 100 kW

1. (a) 37

(b) 49

2. (a) 26

(b) 710

3. (a) 2

(b) 35

4. (a) 53

(b) 73

(b) 0.54 × 10−3 A or 540 × 10−6 A = 0.54 mA or 540 µA

5. (a) 76

(b) 36

6. (a) 1515

(b) 178

(c) 1.5 × 106 = 1.5 M

(d) 22.5 × 10−3 V = 22.5 mV

7. (a) 2

(b) 36

8. (a) 52

(b) 132

(e) 35 × 109 Hz = 35 GHz

(f ) 15 × 10−12 F = 15 pF

9. (a) 34

(b) 1

10. (a) 52

(b)

(g) 17 × 10−6 A = 17 µA

(h) 46.2 × 103 = 46.2 k

12. (a) 1

(b) 1

11. (a) 72

(b)

1 2

1 35

2. (a) 25 µA

3. (a) 13.5 × 10−3

Exercise 9 (Page 17)

2. (a)

32 25

3. (a) 9 4.

(b)

8. −5

65 72

5. −1

(d) 1.25 M

(b) 4 × 103

1. order of magnitude error 2. Rounding-off error – should add ‘correct to 4 significant figures’ or ‘correct to 1 decimal place’

1 210 × 52

(b) ±3 (c) ±

147 148

(c) 620 kV

Exercise 13 (Page 22)

1 (b) 3 7 × 37

1 1. (a) 3 × 52

(b) 1 nF

1 2

19 56

(d) ±

3. Blunder

2 3

4. Measured values, hence c = 55 800 Pa m3 13 45

6. −3

9. 64

10. 4

7.

1 9

1 2

5. Order of magnitude error and rounding-off error – should be 0.022 5, correct to 3 significant figures or 0.022 5, correct to 4 decimal places 6. ≈30 (29.61, by calculator) 7. ≈2 (1.988, correct to 4 s.f., by calculator)

Exercise 10 (Page 18)

8. ≈10 (9.481, correct to 4 s.f., by calculator)

1. (a) 7.39 × 10

(b) 2.84 × 10

(c) 1.976 2 × 102

2. (a) 2.748 × 103

(b) 3.317 × 104

(c) 2.742 18 × 105

Exercise 14 (Page 24)

3. (a) 2.401 × 10−1 (b) 1.74 × 10−2

(c) 9.23 × 10−3

1. (a) 10.56

(b) 5443

(c) 96 970

(d) 0.004 083

4. (a) 1.702 3 × 103 (b) 1.004 × 10

(c) 1.09 × 10−2

2. (a) 2.176

(b) 5.955

(c) 270.7

(d) 0.160 0

5. (a) 5 × 10−1

(b) 1.187 5 × 10

(c) 1.306 × 10

2

6. (a) 1010 7. (a) 0.038 9

(d) 3.125 × 10

(b) 932.7

3. (a) 0.128 7

(b) 0.020 64

(c) 12.25

(d) 0.894 5

−2

(c) 54 100

(b) 0.674 1

4. (a) 558.6

(b) 31.09

5. (a) 109.1

(d) 7

(c) 0.008

6. (a) 0.248 9 7. (a) 0.057 77

(b) 500.5 (b) 28.90

(c) 0.006 761

(b) 3.641

272

Basic Engineering Mathematics

8. (a) 2 515

(b) 146.0

(c) 0.000 029 32

Exercise 18 (Page 32)

9. (a) 18.63

(b) 87.36

(c) 0.164 3

1. (a) 1012

10. (a) 0.005 559

(d) 0.064 56

(b) 1.900

(b) 11112

2. (a) 11 1112

(b) 101 0102

11. (a) 6.248

(b) 0.963 0

3. (a) 0.012

12. (a) 1.165

(b) 2.680

4. (a) 101 111.011 012

13. (a) 1.605

(b) 11.74

(c) 110 101.111 012

14. (a) 6.874 × 10−3

(b) 8.731 × 10−2

(b) 0.001 112

(c) £78

2. (a) 381 mm (b) 56.35 km/h (d) 52 lb 13 oz (e) 6.82 kg

(d) £126

(e) $132

(c) 378.35 km (f) 54.55 litres

(g) 5.5 gallons 3. (a) 7.09 a.m. (b) 51 minutes, 32 m.p.h.

(c) 111 0012

(d) 111 1112

(c) 0.010 012

(d) 0.10 0112

(b) 11 110.110 12 (d) 111 101.101 012

(b) 1 000 111 1002

1. (a) 101 010 1112

(b) $175.78

(d) 11 1012

Exercise 19 (Page 33)

Exercise 15 (Page 25) 1. (a) 49.40 euro

(c) 10 0112

(c) 7.04 a.m.

2. (a) 0.011 112

(b) 0.101 12

3. (a) 11 110 111.000 112

(c) 10 011 110 0012

(c) 0.101 112

(b) 1 000 000 010.011 12

(c) 11 010 110 100.110 012 4. (a) 7.437 510

(b) 41.2510

(c) 7 386.187 510

Exercise 20 (Page 36)

Exercise 16 (Page 28) 1. A = 66.59 cm2

2. C = 52.78 mm

1. 23110

2. 4410

3. 15210

4. 75310

3. R = 37.5

4. 159 m/s

5. 3616

6. C816

7. 5B16

8. EE16

5. 0.407 A

6. 0.005 02 m or 5.02 mm

9. D716

10. EA16

11. 8B16

12. A516

7. 0.144 J

8. 628.8 m2

13. 110 1112 3

9. 224.5

10. 14 230 kg/m

11. 281.1 m/s

12. 2.526

13. £589.27

14. 508.1 W

15. V = 2.61 V

16. F = 854.5

17. I = 3.81 A

18. T = 14.79 s

19. E = 3.96 J

20. I = 12.77 A

21. S = 17.25 m

22. A = 7.184 cm2

(b) 2510

3. (a) 0.812 510 (c) 0.218 7510 4. (a) 26.7510 (c) 53.437 510

1. −16

2. −8

1 4. 2 c 6

1 1 5. −2 x + 1 y + 2z 4 2

6. a − 4b − c

7. 9d − 2e

3. 4a

8. 3x − 5y + 5z

10. 3x2 − xy − 2y2

11. 6a2 − 13ab + 3ac − 5b2 + bc

Exercise 17 (Page 30)

2. (a) 2110

16. 101 000 100 0012

5 1 9. −5 a + b − 4c 2 6

(b) 1110

15. 10 011 1112

Exercise 21 (Page 39)

23. v = 7.327

1. (a) 610

14. 11 101 1012

(c) 1410

(d) 910

(c) 4510

(d) 5110

(b) 0.781 2510 (d) 0.343 7510 (b) 23.37510 (d) 213.718 7510

12. (i)

1 3b

13. 2x − y

(ii) 2ab

14. 3p + 2q

Exercise 22 (Page 41) 1. x5 y4 z 3 , 13

1 2

4. x7/10 y1/6 z 1/2

2. a2 b1/2 c−2 , 4

5.

1+a b

1 2

3. a3 b−2 c, 9

6.

p2 q q−p

Answers to exercises

√ 6 9. xy3 z 13

8. a−4 b5 c11

7. ab6 c3/2

√ 6

1 10. ef 2

11. a11/6 b1/3 c−3/2 or

Exercise 26 (Page 48)

√ 3 a11 b √ c3

1. 1

2. 2

3. 6

6. 1

7. 2

8.

4. −4

1 2

11. 2 12. −10 13. 6

Exercise 23 (Page 43)

9. 0

10. 3

14. −2

15. 2 12

18. −3

17. 6 14

1. 3x + y

2. 3a + 5y

4. x(3x − 3 + y)

5. −5p + 10q − 6r

6. a2 + 3ab + 2b2

7. 3p2 + pq − 2q2

1. 5

2. −2

3. −4 12

4. 2

8. (i) x2 − 4xy + 4y2

(ii) 9a2 − 6ab + b2

5. 12

6. 15

7. −4

8. 5 13

9. 2

10. 13

11. −10

12. 2

13. 3

14. −11

15. −6

16. 9

17. 6 14

18. 3

19. 4

20. 10

21. ±12

22. −3 13

23. ±3

24. ±4

9. 3ab + 7ac − 4bc 12. 2 + 5b

3. 5(x − y)

5. 1 23

11. 4 − a

10. 0

13. 11q − 2p

2

14. (i) p(b + 2c)

(ii) 2q(q + 4n)

15. (i) 7ab(3ab − 4)

(ii) 2xy(y + 3x + 4x ) 2

16. 2

Exercise 27 (Page 50)

16. (i) (a + b)(y + 1) (ii) (p + q)(x + y) 17. (i) (x − y)(a + b) (ii) (a − 2b)(2x + 3y) Exercise 24 (Page 44) 1.

1 + 6x 2

1 5

4. a(4a + 1)

5. a(3 − 10a)

6.

2 − 3y + 12 3y

8.

2 + 12 − 13y 3y

11.

1 (x − 4) 2

12. y

1 + 3y 2

(b) 78

2. (a) 0.0075

(b) 3.15 litres

3. (a) 0.000 08 4. (a) 9.18

5. (a) 300 × 103 6. 1.5 kW

(c) 350 K −3

(b) 4.16 × 10 A (c) 45 V

(b) 6.12

3. 3.472

5. 12p, 17p

6. 800

1. 12 cm, 240 cm2

2. 0.004

3. 30

4. 45◦ C

5. 50

6. £208, £160

7. 30 kg

8. 12 m, 8 m 9. 3.5 kN

Exercise 30 (Page 55) 10. pq

Exercise 25 (Page 46) 1. (a) 15

(b) 30

2. 8 m/s2

Exercise 29 (Page 53)

3. 4a(1 − 2a)

2 1 + 12 − 5y 3y 3 5 9. + 1 y

1. 10−7 4. (a) 1.8

2.

7.

Exercise 28 (Page 51)

c 1 1. d = c − a − b 2. y = (t − x) 3. r = 3 2π 4. x =

y−c m

7. R =

a S −a or 1 − S S

(c) 24 × 103

I PR

6. R =

E I

5 8. C = (F − 32) 9

Exercise 31 (Page 57) yd λ

1. x =

d (y + λ) λ

or

2. f =

3F − AL 3

or f = F −

4. t =

R − R0 R0 α

(c) 0.3375

(b) 0.375 m2

5. T =

d+

5. R2 =

AL 3 RR1 R1 − R

3. E =

Ml 2 8yI

273

274

Basic Engineering Mathematics

E − e − IR I y

7. b = 4ac2

or R =

6. R =

10. u =

E−e −r I

Exercise 35 (Page 65) t2g 9. l = 2 4π

ay

8. x = (y2 − b2 ) 360A 11. R = πθ √ Z 2 − R2 13. L = 2πf

√ v 2 − 2as

12. a = N y − x 2

1. x = 12 , y = 14

2.

3. p = 14 , q = 15

4. x = 10, y = 5

5. c = 3, d = 4

6. r = 3, s = 12

7. x = 5, y = 1 34

8. 1

1 , 3

b = − 12

Exercise 36 (Page 68) Exercise 32 (Page 58) xy 1. a = m−n 3(x + y) 3. r = (1 − x − y)

a(p2 − q2 ) 2(p2 + q2 )

9. t2 = t1 + 11. l =

2. R =

4

M + r4 π

mrCR 4. L = µ−m x−y 6. r = x+y

c 5. b = √ 1 − a2 7. b =

uf , 30 u−f 2dgh 10. v = , 0.965 0.03L

12. C =

ω{ωL −

Z 2 − R2 }

, 63.1 × 10−6

14. λ = 5

13. 64 mm

3. u = 12, a = 4, v = 26

4. £15 500, £12 800

5. m = − 12 , c = 3

6. α = 0.00 426, R0 = 22.56

7. a = 12, b = 0.40

8. a = 4, b = 10

1. 4, −8

aµ ρCZ 4 n

2

2. 4, −4

3. 2, −6

6.

1 , 2

− 45

8. 1 13 , − 17

9.

3 , 8

−2

10.

12.

5 , 4

− 32

13. x2 − 4x + 3 = 0

4 , 3

− 12

7. 2 2 , 5

−3

14. x2 + 3x − 10 = 0

15. x2 + 5x + 4 = 0

16. 4x2 − 8x − 5 = 0

17. x2 − 36 = 0

18. x2 − 1.7x − 1.68 = 0 Exercise 38 (Page 72)

Exercise 33 (Page 62)

1. −3.732, −0.268

2. −3.137, 0.637

1. a = 5, b = 2

3. 1.468, −1.135

4. 1.290, 0.310

5. 2.443, 0.307

6. −2.851, 0.351

2. x = 1, y = 1

3. s = 2, t = 3 5.

m = 2 12 ,

n=

4. x = 3, y = −2 1 2

7. x = 2, y = 5

6. a = 6, b = −1 8. c = 2, d = −3

4. −1, 1 12

1 1 , 2 3

5.

11.

8S 2 + d, 2.725 3d 1 √

2. I1 = 6.47, I2 = 4.62

Exercise 37 (Page 70)

8. v =

Q , 55 mc

1. a = 15 , b = 4

Exercise 39 (Page 73) 1. 0.637, −3.137

2. 0.296, −0.792

3. 2.781, 0.719

4. 0.443, −1.693

5. 3.608, −1.108

6. 4.562, 0.438

Exercise 34 (Page 63) 1. p = −1, q = −2

2. x = 4, y = 6

3. a = 2, b = 3

4. s = 4, t = −1

5. x = 3, y = 4

6. u = 12, v = 2

Exercise 40 (Page 75)

7. x = 10, y = 15

8. a = 0.30, b = 0.40

1. 1.191 s

2. 0.345 A or 0.905 A

3. 7.84 cm

4. 0.619 m or 19.38 m

Answers to exercises

5. 0.013 3

6. 1.066 m

7. 86.78 cm

8. 1.835 m or 18.165 m

9. 7 m

7. t ≥

8. k ≥

10. 12 ohms, 28 ohms

Exercise 41 (Page 76) 1. x = 1, y = 3

and

2. x = 25 , y = − 15 3. x = 0, y = 4

x = −3, y = 7

1. 14.5

x = 3, y = 1

13 1 − − 4 2

or k ≤

(b) x < 5

2. (a) x > 3

3. (a) t ≤ 1

(b) x ≤ 6

4. (a) k ≥

(b) x ≥ −

(b) −1, 0

(c) −3, −4

(b) 3, −2 12

(c)

5. (a) 6, 3

(b) −2, 4

(c) 3, 0

6. (a) 2, − 12

(b) − 23 , −1 23

(c)

(b) −4

(c) −1 56

4. (a) 2,

1. (a) t > 2

4 3

1 2

2.

3. (a) 4, −2

Exercise 42 (Page 78)

5. (a) y ≥ −

13 1 − 4 2

Exercise 47 (Page 88)

and −1 23 , y = −4 13 and

√

√

11 + 2 or t ≤ 2 − 11

(b) x ≥ 3

3 2

(b) z >

1 2

7. (a)

1 2

1 2

3 5

8. (a) and (c), (b) and (e)

(d) 0, 4

1 1 , 24 2

1 , 18

(d) 0, 7 (d) 10, −4 23

2

9. (a) −1.1

(b) −1.4

11. 1 12 , 6

10. (2, 1) Exercise 43 (Page 78) 1. −5 < t < 3

2. −5 ≤ y ≤ −1

1 4. t > 3 and t < 3

5. k ≥ 4 and k ≤ −2

5 3 3. − < x < 2 2

1. (a) 40◦ C

(b) 128

2. (a) 850 rev/min 3. (a) 0.25

Exercise 44 (Page 79) 1. −4 ≤ x < 3

Exercise 48 (Page 92)

(c) F = 0.25L + 12

(b) 12

(d) 89.5 N

2. t > 5 or t < −9

(b) 77.5 V

(e) 592 N

(f) 212 N

4. −0.003, 8.73

3. −5 < z ≤ 14 4. −3 < x ≤ −2

5. (a) 22.5 m/s (b) 6.43 s (c) v = 0.7t + 15.5 Exercise 45 (Page 80) 1. z > 4 or z < −4 3. x ≥

√ √ 3 or x ≤ − 3

5. −5 ≤ t ≤ 7

6. m = 26.9L − 0.63

2. −4 < z < 4

7. (a) 1.26 t

4. −2 ≤ k ≤ 2

8. (a) 96 × 109 Pa

6. t ≥ 7 or t ≤ −5

9. (a) 7. y ≥ 2 or y ≤ −2

1 8. k > − or k < −2 2

1 5

(b) 6

(c) F = −0.09w + 2.21

(b) 0.000 22

(c) E = 15 L + 6

(c) 28.8 × 106 Pa (d) 12 N

Exercise 49 (Page 95)

1. x > 3 or x < −2

2. −4 ≤ t ≤ 2

4. y ≥ 5 or y ≤ −4

5. −4 ≤ z ≤ 0

√

√ 3−3 − 3−3 ≤x≤

3. −2 < x <

(e) 65 N

10. a = 0.85, b = 12, 254.3 kPa, 275.5 kPa, 280 K

Exercise 46 (Page 81)

6.

(b) 21.68%

1 2

1. x = 1, y = 1

2. x = 3 12 , y = 1 12

3. x = −1, y = 2

4. x = 2.3, y = −1.2

5. x = −2, y = −3

6. a = 0.4, b = 1.6

275

276

Basic Engineering Mathematics

19. 2 log 2 + log 3 + log 5

Exercise 50 (Page 99) 1. (a) Minimum (0, 0) (b) Minimum (0, −1)

20. 2 log 2 +

(c) Maximum (0, 3) (d) Maximum (0, −1) 2. −0.4 or 0.6

3. −3.9 or 6.9

4. −1.1 or 4.1

5. −1.8 or 2.2

21. 4 log 2 − 3 log 3 + 3 log 5 22. log 2 − 3 log 3 + 3 log 5

6. x = −1 12 or −2, Minimum at (−1 34 , − 18 ) 7. x = −0.7 or 1.6 8. (a) ±1.63

24. 4 log 2

25. 6 log 2

26.

or

− 13

9. (−2.58, 13.31), (0.58, 0.67); x = −2.58 10. x = −1.2

23. 5 log 3

28. x = 2

(b) 1

(b) 2.75 and −1.45

1 2

1 2

29. t = 8

3 2

27.

30. b = 2

Exercise 54 (Page 106) or

(a) −30

or 2.5

1 log 5 − 3 log 3 4

(c) 2.29 or −0.79

0.58 1. 1.691

2. 3.170

3. 0.2696

4. 6.058

5. 2.251

6. 3.959

7. 2.542

8. −0.3272

Exercise 51 (Page 100) Exercise 55 (Page 108)

1. x = 4, y = 8 and x = − 12 , y = −5 12 2. (a) x = −1.5 or 3.5

(b) x = −1.24 or 3.24

(c) x = −1.5 or 3.0

1. (a) 81.45

(b) 0.7788

(c) 2.509

2. (a) 0.1653

(b) 0.4584

(c) 22030

3. (a) 57.556

(b) −0.26776

(c) 645.55

4. (a) 5.0988

(b) 0.064037

(c) 40.446

5. (a) 4.55848

(b) 2.40444

(c) 8.05124

Exercise 52 (Page 101) 1. x = −2.0, −0.5 or 1.5 2. x = −2, 1 or 3, Minimum at (2.12, −4.10),

6. (a) 48.04106

Maximum at (−0.79, 8.21)

(b) 4.07482

3. x = 1

4. x = −2.0, 0.38 or 2.6

7. 2.739

5. x = 0.69 or 2.5

6. x = −2.3, 1.0 or 1.8

Exercise 56 (Page 109)

8. 120.7 m

7. x = −1.5

1. 2.0601

Exercise 53 (Page 105)

8 3. 1 − 2x2 − x3 − 2x4 3

1. 4 1 3 1 9. 1 2 5.

2. 4

3. 3

4. −3

6. 3

7. 2

8. −2

2. (a) 7.389

(b) 0.7408

1 1 1 4. 2x1/2 + 2x5/2 + x9/2 + x13/2 + x17/2 + x21/2 3 12 60 Exercise 57 (Page 111)

1 10. 3

11. 2

13. 100 000 14. 9

15. ±

16. 0.01

(c) −0.08286

17.

1 16

18. e3

12. 10 000 1 32

1. 3.97, 2.03

2. 1.66, −1.30

3. (a) 27.9 cm3

(b) 115.5 min

4. (a) 71.6◦ C

(b) 5 minutes

Answers to exercises

3. (a) ln y

Exercise 58 (Page 113)

(b) x

(c) n

(d) ln m

1. (a) 0.5481

(b) 1.6888

(c) 2.2420

4. I = 0.0012 V2 , 6.75 candelas

2. (a) 2.8507

(b) 6.2940

(c) 9.1497

5. a = 3.0, b = 0.5

3. (a) −1.7545 (b) −5.2190 4. (a) 0.27774

(b) 0.91374

(c) −2.3632 (c) 8.8941

277

6. a = 5.7, b = 2.6, 38.53, 3.0 7. R0 = 26.0, c = 1.42 8. Y = 0.08e0.24x

5. (a) 3.6773

(b) −0.33154

(c) 0.13087

6. −0.4904

7. −0.5822

8. 2.197

9. T0 = 35.4 N, µ = 0.27, 65.0 N, 1.28 radians Exercise 62 (Page 127)

9. 816.2

10. 0.8274

1. a = 12, n = 1.8, 451, 28.5 2. k = 1.5, n = −1

Exercise 59 (Page 115) (b) 100.5◦ C

1. 150◦ C

3. m = 1, n = 10

2. 9.921 × 104 Pa Exercise 63 (Page 128)

3. (a) 29.32 volts (b) 71.31 × 10−6 s

1. (i) a = −8, b = 5.3, p = −8(5.3)q

(ii) −224.7

(iii) 3.31

4. (a) 1.993 m (b) 2.293 m Exercise 64 (Page 130) 5. (a) 50◦ C

(b) 55.45 s

6. 30.4 N 8. 2.45 mol/cm

3

1. a = 76, k = −7 × 10−5 , p = 76e−7×10

7. (a) 3.04 A

(b) 1.46 s

9. (a) 7.07 A

(b) 0.966 s

−5 h

, 37.74 cm

2. θ0 = 152, k = −0.05 Exercise 65 (Page 132)

10. £2424

1. (a) 82◦ 11

Exercise 60 (Page 119)

2. (a) 7◦ 11

1. (a) y

(b) x2

2. (a) y

(b)

3. (a) y

(b)

y x y 5. (a) x 4. (a)

√

(c) c x

(c) f

(d) e

(b) x

(c) b

(d) c

1 (b) 2 x

(c) a

(c) 100◦ 16 3

(c) 18◦ 47 49

(b) 29.883◦

4. (a) 25◦ 24 0

(d) a

1 x

(b) 27◦ 48

3. (a) 15.183◦

(d) d

(c) b

(b) 150◦ 13

(c) 49.705◦

(d) 89◦ 2 23

(d) 66◦ 7 23

(d) 135.122◦

(b) 36◦ 28 48

(c) 55◦ 43 26

(d) 231◦ 1 30

Exercise 66 (Page 133) 1. (a) acute

(d) b

6. a = 1.5, b = 0.4, 11.78 mm

(b) obtuse

(c) reflex

2. (a) 21◦

(b) 62◦ 23

(c) 48◦ 56 17

3. (a) 102◦

(b) 165◦

(c) 10◦ 18 49

2

7. y = 2x2 + 7, 5.15 9. a = 0.4, b = 8.6

4. (a) 1 & 3, 2 & 4, 5 & 7, 6 & 8

8. (a) 950 (i) 94.4

(b) 317 kN

(ii) 11.2

(b) 1 & 2, 2 & 3, 3 & 4, 4 & 1, 5 & 6, 6 & 7, 7 & 8, 8 & 5, 3 & 8, 1 & 6, 4 & 7 or 2 & 5 (c) 1 & 5, 2 & 6, 4 & 8, 3 & 7

Exercise 61 (Page 122) 1. (a) lg y

(b) x

2. (a) lg y

(b) lg x

(d) 3 & 5

(c) lg a (d) lg b (c) l

(d) lg k

5. 59◦ 20

or

2 & 8

6. a = 69◦ , b = 21◦ , c = 82◦

7. 51◦

278

Basic Engineering Mathematics

Exercise 67 (Page 135)

3. sin A =

8 8 , tan A = 17 15

4. sin X =

15 112 , cos X = 113 113

1. 40◦ , 70◦ , 70◦ , 125◦ , isosceles ◦

◦

◦

2. a = 18 50 , b = 71 10 , c = 68 , d = 90◦ , e = 22◦ , f = 49◦ , g = 41◦

5. (a)

3. a = 103◦ , b = 55◦ , c = 77◦ , d = 125◦ , e = 55◦ , f = 22◦ ,

(b)

6. (a) sin θ =

g = 103◦ , h = 77◦ , i = 103◦ , j = 77◦ , k = 81◦ 4. 17◦

15 17

5. A = 37◦ , B = 60◦ , E = 83◦

7. (a) 9.434

Exercise 68 (Page 137)

15 17

7 25

(c)

8 15 24 25

(b) cos θ =

(b) −0.625

(c) −32◦

Exercise 73 (Page 146)

1. (a) Congruent BAC, DAC

(SAS)

1. BC = 3.50 cm, AB = 6.10 cm, ∠B = 55

(b) Congruent FGE, JHI

(SSS)

2. FE = 5 cm, ∠E = 53◦ 8 , ∠F = 36◦ 52

(c) Not necessarily congruent

3. GH = 9.841 mm, GI = 11.32 mm, ∠H = 49◦

(d) Congruent QRT , SRT

(RHS)

4. KL = 5.43 cm, JL = 8.62 cm, ∠J = 39◦ , area = 18.19 cm2

(e) Congruent UVW , XZY

(ASA)

5. MN = 28.86 mm, NO = 13.82 mm, ∠O = 64◦ 25 , area = 199.4 mm2

2. Proof

6. PR = 7.934 m, ∠Q = 65◦ 3 , ∠R = 24◦ 57 , area = 14.64 m2

Exercise 69 (Page 139) 1. x = 16.54 mm, y = 4.18 mm 3. (a) 2.25 cm

(b) 4 cm

7. 6.54 m

2. 9 cm, 7.79 cm

4. 3 m

8. 9.40 mm

Exercise 74 (Page 148)

Exercise 70 (Page 140)

1. 36.15 m

2. 48 m

3. 249.5 m

1.–5. Constructions – see similar constructions in worked problems 23 to 26 on pages 139 and 140.

4. 110.1 m

5. 53.0 m

6. 9.50 m

7. 107.8 m

8. 9.43 m, 10.56 m

9. 60 m

Exercise 71 (Page 143)

Exercise 75 (Page 150)

1. 11.18 cm

2. 24.11 mm

4. (a) 27.20 cm each (b) 45◦ 6. 3.35 m, 10 cm

7. 132.7 km

8. 2.94 mm

9. 24 mm

3. 8 + 15 = 17

1. (a) 0.4540

5. 20.81 km

2. (a) −0.5592

(b) 0.9307

3. (a) −0.7002

(b) −1.1671

2

2

2

4. (a) 0.8660

Exercise 72 (Page 145) 40 40 9 9 1. sin Z = , cos Z = , tan X = , cos X = 41 41 9 41 4 3 4 3 3 2. sin A = , cos A = , tan A = , sin B = , cos B = , 5 5 4 5 5 4 tan B = 3

(b) 0.1321

(b) −0.1010

(c) −0.8399 (c) 0.2447 (c) 1.1612 (c) 0.5865

5. 13.54◦ , 13◦ 32 , 0.236 rad 6. 34.20◦ , 34◦ 12 , 0.597 rad 7. 39.03◦ , 39◦ 2 , 0.681 rad 8. 1.097 11. 21◦ 42

9. 5.805

10. −5.325

12. 0.07448

13. (a) −0.8192

(b) −1.8040

(c) 0.6528

Answers to exercises

7. (0.750, −1.299)

Exercise 76 (Page 154) ◦

◦

(b) 188.53 and 351.47

2. (a) 29.08◦ and 330.92◦

(b) 123.86◦ and 236.14◦

3. (a) 44.21◦ and 224.21◦

(b) 113.12◦ and 293.12◦

1. (a) 42.78 and 137.22

◦

◦

8. (4.252, −4.233)

9. (a) 40 ∠18◦ , 40 ∠90◦ , 40 ∠162◦ , 40 ∠234◦ , 40 ∠306◦ (b) (38.04 + j12.36), (0 + j40), (−38.04 + j12.36), (−23.51 − j32.36), (23.51 − j32.36) Exercise 81 (Page 170)

Exercise 77 (Page 158) 1. 1, 120◦ 5.

7 , 2

2. 2, 144◦

960◦ 6. 6, 360◦

1. 35.7 cm2

4. 3, 720◦

3. 3, 90◦

2. (a) 80 m

3. (a) 29 cm2

7. 4, 180◦

(b) 170 m

(b) 650 mm2

4. 482 m2

6. p = 105◦ , q = 35◦ , r = 142◦ , s = 95◦ , t = 146◦

Exercise 78 (Page 160)

(a) 14 cm2

7. (i) rhombus ◦

1. 40, 0.04 s, 25 Hz, 0.29 rad (or 16 37 ) leading 40 sin 50πt 2. 75 cm, 0.157 s, 6.37 Hz, 0.54 rad (or 30◦ 56 ) lagging 75 sin 40t 3. 300 V, 0.01 s, 100 Hz, 0.412 rad (or 23◦ 36 ) lagging 300 sin 200πt 4. (a) v = 120 sin 100πt volts

(ii) parallelogram (iii) rectangle

(a) 3600 mm (a) 190 cm2

8. (a) 50.27 cm2

(b) 706.9 mm2

(b) 80 mm 2

(b) 300 mm (b) 62.91 cm

(c) 3183 mm2

9. 2513 mm2 (b) 63.41 mm

2

(b) 129.9 mm2

11. (a) 53.01 cm 12. 5773 mm2

6. 3.2 sin(100πt + 0.488) m

(a) 180 mm

(b) 16 cm 2

(iv) trapezium

10. (a) 20.19 mm

(b) v = 120 sin (100πt + 0.43) volts π

5. i = 20 sin 80π t − amperes 6

13. 1.89 m2

(c) 6.84 cm2

14. 6750 mm2

Exercise 82 (Page 172) ◦

5. 3.4 cm

7. (a) 5 A, 20 ms, 50 Hz, 24 45 lagging (c) 4.363 A (d) 6.375 ms (e) 3.423 ms

(b) −2.093 A

1. 1932 mm2

2. 1624 mm2

3. (a) 0.918 ha

(b) 456 m

Exercise 79 (Page 163) 1. (5.83, 59.04◦ ) or (5.83, 1.03 rad)

4. 32

2. (6.61, 20.82◦ ) or (6.61, 0.36 rad)

Exercise 83 (Page 172)

3. (4.47, 116.57◦ ) or (4.47, 2.03 rad)

1. 80 ha 2. 80 m2

4. (6.55, 145.58◦ ) or (6.55, 2.54 rad) ◦

5. (7.62, 203.20 ) or (7.62, 3.55 rad) ◦

6. (4.33, 236.31 ) or (4.33, 4.12 rad)

3. 3.14 ha

Exercise 84 (Page 175) 1. 45.24 cm

2. 259.5 mm 3. 2.629 cm

4. 12 730 km

5. 97.13 mm

◦

7. (5.83, 329.04 ) or (5.83, 5.74 rad) 8. (15.68, 307.75◦ ) or (15.68, 5.37 rad)

Exercise 85 (Page 177) 1. (a)

Exercise 80 (Page 165)

π 6

(b)

2. (a) 0.838

5π 12

(c)

(b) 1.481

5π 4 (c) 4.054

1. (1.294, 4.830)

2. (1.917, 3.960)

3. (−5.362, 4.500)

4. (−2.884, 2.154)

3. (a) 150◦

(b) 80◦

5. (−9.353, −5.400)

6. (−2.615, −3.207)

4. (a) 0◦ 43

(b) 154◦ 8

(c) 105◦ (c) 414◦ 53

279

280

Basic Engineering Mathematics

5. 17.80 cm, 74.07 cm2

Exercise 89 (Page 189)

6. (a) 59.86 mm

1. 147 cm3 , 164 cm2

(b) 197.8 mm

2. 403 cm3 , 337 cm2

7. 26.2 cm 8. 8.67 cm, 54.48 cm

3. 10480 m3 , 1852 m2 4. 1707 cm2

9. 82◦ 30

5. 10.69 cm

10. 748

11. (a) 0.698 rad

6. 55910 cm3 , 8427 cm2

7. 5.14 m

(b) 804.2 m2 Exercise 90 (Page 190)

12. 10.47 m2 13. (a) 396 mm2 14. 483.6 mm

1. 8 : 125

(b) 42.24% 15. 7.74 mm

2. 137.2 g

Exercise 91 (Page 192)

Exercise 86 (Page 179)

1. 4.5 square units

2. 54.7 square units

1. (a) 3

4. 4.70 ha

5. 143 m2

(b) (−4, 1)

2. Centre at (3, −2), radius 4

3. 63 m

Exercise 92 (Page 193)

3. Circle, centre (0, 1), radius 5

1. 42.59 m3

2. 147 m3

3. 20.42 m3

4. Circle, centre (0, 0), radius 6 Exercise 93 (Page 196) Exercise 87 (Page 182) 1. (a) 2 A

1. 15 cm3 , 135 g

2. 500 litres

3. 1.44 m3

4. 8796 cm3

5. 4.709 cm, 153.9 cm2

6. 201.1 cm3 , 159.0 cm2

7. 2.99 cm

8. 28060 cm3 , 1.099 m2

(b) 50 V

2. (a) 2.5 V

(c) 2.5 A

(b) 3 A

3. 0.093 As, 3.1 A 4. (a) 31.83 V

(b) 0

2

9. 7.68 cm3 , 25.81 cm2

10. 113.1 cm3 , 113.1 cm2

11. 5890 mm2 or 58.90 cm2 12. 62.5 minutes

5.

2. A = 52◦ 2 , c = 7.568 cm, a = 7.152 cm, area = 25.65 cm2

2. 5.131 cm

3. 29.32 cm3

Exercise 94 (Page 200) 1. C = 83◦ , a = 14.1 mm, c = 28.9 mm, area = 189 mm2

Exercise 88 (Page 185) 1. 13.57 kg

5. 49.13 cm , 368.5 kPa

3. D = 19◦ 48 , E = 134◦ 12 , e = 36.0 cm, area = 134 cm2

4. 393.4 m2

(i)

(a) 670 cm3

(b) 523 cm2

(ii)

(a) 180 cm3

(b) 154 cm2

(iii)

(a) 56.5 cm3

(b) 84.8 cm2

(iv)

(a) 10.4 cm3

(b) 32.0 cm2

(v)

(a) 96.0 cm

3

(vi)

(a) 86.5 cm

3

(vii)

(a) 805 cm3

2

(b) 146 cm

4. E = 49◦ 0 , F = 26◦ 38 , f = 15.08 mm, area = 185.6 mm2 5. J = 44◦ 29 , L = 99◦ 31 , l = 5.420 cm, area = 6.132 cm2 OR J = 135◦ 31 , L = 8◦ 29 , l = 0.810 cm, area = 0.916 cm2 6. K = 47◦ 8 , J = 97◦ 52 , j = 62.2 mm, area = 820.2 mm2 OR K = 132◦ 52 , J = 12◦ 8 , j = 13.19 mm, area = 174.0 mm2

2

(b) 142 cm

(b) 539 cm2

6. 8.53 cm

7. (a) 17.9 cm

(b) 38.0 cm

8. 125 cm3

9. 10.3 m3 , 25.5 m2

Exercise 95 (Page 201) 1. p = 13.2 cm, Q = 47◦ 21 , R = 78◦ 39 , area = 77.7 cm2 2. p = 6.127 m, Q = 30◦ 49 , R = 44◦ 11 , area = 6.938 m2

10. 6560 litres

11. 657.1 cm3 , 1027 cm2

3. X = 83◦ 20 , Y = 52◦ 37 , Z = 44◦ 3 , area = 27.8 cm2

12. 220.7 cm2

13. (a) 1458 litre (b) 9.77 m2 (c) £140.45

4. Z = 29◦ 46 , Y = 53◦ 31 , Z = 96◦ 43 , area = 355 mm2

Answers to exercises

Exercise 96 (Page 203)

3. i = 21.79 sin(ωt − 0.639) A

1. 193 km

4. v = 5.695 sin(ωt + 0.670) V

2. (a) 122.6 m (b) 94◦ 49 , 40◦ 39 , 44◦ 32

5. x = 14.38 sin(ωt + 1.444) cm

3. (a) 11.4 m

6. 13 sin(ωt + 1.176) mA

(b) 17◦ 33

4. 163.4 m

Exercise 103 (Page 218)

5. BF = 3.9 m, EB = 4.0 m

1. 21, 25

2. 48, 96

5. 50, 65

6. 0.001, 0.0001

6. 6.35 m, 5.37 m

7. 32.48 A, 14◦ 19

3. 14, 7

4. −3, −8

7. 54, 79

Exercise 104 (Page 219)

Exercise 97 (Page 205) 1. 80◦ 25 , 59◦ 23 , 40◦ 12

2. (a) 15.23 m

(b) 38◦ 4

3. 40.25 cm, 126◦ 3

4. 19.8 cm

5. 36.2 m

6. x = 69.3 mm, y = 142 mm

7. 130◦

8. 13.66 mm

1. 1, 3, 5, 7, …

2. 7, 10, 13, 16, 19, …

3. 6, 11, 16, 21, …

4. 5n

5. 6n − 2

6. 2n + 1

7. 4n − 2

8. 3n + 6

9. 63 (= 216), 73 (= 343) Exercise 105 (Page 220)

Exercise 98 (Page 210)

1. 68

2. 6.2

3. 85.25

5. 11

6. 209

7. 346.5

1. 47 N at 29◦

2. Zero

3. 7.27 m/s at 90.8◦

4. 6.24 N at 76.10◦

5. 2.46 N, 4.12 N

6. 5.7 m/s2 at 310◦

Exercise 106 (Page 222)

7. 11.85 A at 31.14◦

8. 15.62 N at 26.33◦ to the 10 N force

1. − 12 5.

9. 8.50 km/h at 41.73◦ E of S

8 12 ,

2. 1 12 , 3, 4 12 12,

6. (a) 120

4. 25

(b) 26070

(c) 250.5 8. £8720

(b) 45.64 N at 4.66◦

2. (a) 31.71 m/s at 121.81◦ (b) 19.55 m/s at 8.63◦ Exercise 100 (Page 213) 1. 83.5 km/h at 71.6◦ to the vertical 2. 4 minutes 55 seconds

3. 7808

19

7. £10000, £109500

Exercise 99 (Page 212) 1. (a) 54.0 N at 78.16◦

15 12 ,

4. 23 12

3. 23.18 m/s at 72.24◦ E of S

Exercise 107 (Page 223) 1. 2560

2. 273.25

3. 512, 4096

5. 812.5

6. 8

7. 1 23

4. 10th

Exercise 108 (Page 225) 1. (a) 3

(b) 2

(c) 59022

2. £1566, 11 years

Exercise 101 (Page 215)

3. 48.71 M 4. 71.53 g 5. (a) £599.14

1. 4.5 sin(A + 63.5◦ )

6. 100, 139, 193, 268, 373, 518, 720, 1000 rev/min

2. (a) 20.9 sin(ωt + 0.62) volts 3. 13 sin(ωt + 0.395)

(b) 12.5 sin(ωt − 1.33) volts

(b) 19 years

Exercise 109 (Page 226) 1. (a) continuous

(b) continuous (c) discrete

(d) continuous Exercise 102 (Page 217) 1. 11.11 sin(ωt + 0.324) 2. 8.73 sin(ωt − 0.173)

2. (a) discrete (d) discrete

(b) continuous

(c) discrete

281

282

Basic Engineering Mathematics

Exercise 110 (Page 229) 1. If one symbol is used to represent 10 vehicles, working correct to the nearest 5 vehicles, gives 3 12 , 4 12 , 6, 7, 5 and 4 symbols respectively 2. If one symbol represents 200 components, working correct to the nearest 100 components gives: Mon 8, Tues 11, Wed 9, Thurs 12 and Fri 6 12 3. 6 equally spaced horizontal rectangles, whose lengths are proportional to 35, 44, 62, 68, 49 and 41, respectively 4. 5 equally spaced horizontal rectangles, whose lengths are proportional to 1580, 2190, 1840, 2385 and 1280 units, respectively 5. 6 equally spaced vertical rectangles, whose heights are proportional to 35, 44, 62, 68, 49 and 41 units, respectively 6. 5 equally spaced vertical rectangles, whose heights are proportional to 1580, 2190, 1840, 2385 and 1280 units, respectively 7. Three rectangles of equal height, subdivided in the percentages shown in the columns of the question. P increases by 20% at the expense of Q and R 8. Four rectangles of equal height, subdivided as follows: week 1: 18%, 7%, 35%, 12%, 28%

2. Rectangles, touching one another, having mid-points of 39.35, 39.55, 39.75, 39.95, . . . and heights of 1, 5, 9, 17, . . . 3. There is no unique solution, but one solution is: 20.5 − 20.9 3; 21.0 − 21.4 22.0 − 22.4

10; 21.5 − 21.9

13; 22.5 − 22.9

9; 23.0 − 23.4

11; 2

4. There is no unique solution, but one solution is: 1 − 10

3; 11 − 19

26 − 28

7; 29 − 38

7; 20 − 22 5; 39 − 48

12; 23 − 25

14;

2

5. 20.95 3; 21.45 13; 21.95 24; 22.45 37; 22.95 46; 23.45 48 6. Rectangles, touching one another, having mid-points of 5.5, 15, 21, 24, 27, 33.5 and 43.5. The heights of the rectangles (frequency per unit class range) are 0.3, 0.78, 4, 4.67, 2.33, 0.5 and 0.2 7. (20.95 3), (21.45 13), (21.95 24), (22.45 37), (22.95 46), (23.45 48) 8. A graph of cumulative frequency against upper class boundary having co-ordinates given in the answer to question 7 9. (a) There is no unique solution, but one solution is: 2.05 − 2.09

3; 2.10 − 2.14

10; 2.15 − 2.19

2.20 − 2.24

13; 2.25 − 2.29

9; 2.30 − 2.34

11; 2

(b) Rectangles, touching one another, having mid-points of 2.07, 2.12, . . . and heights of 3, 10, . . .

week 2: 20%, 8%, 32%, 13%, 27% week 3: 22%, 10%, 29%, 14%, 25% week 4: 20%, 9%, 27%, 19%, 25%

(c) Using the frequency distribution given in the solution to part (a) gives: 2.095 3; 2.145 13; 2.195 24; 2.245 37; 2.295 46; 2.345 48

Little change in centres A and B, a reduction of about 5% in C, an increase of about 7% in D and a reduction of about 3% in E

(d) A graph of cumulative frequency against upper class boundary having the co-ordinates given in part (c)

9. A circle of any radius, subdivided into sectors having angles ◦ ◦ ◦ ◦ of 7 12 , 22 12 , 52 12 , 167 12 and 110◦ , respectively

Exercise 112 (Page 236) 1. Mean 7 13 , median 8, mode 8

10. A circle of any radius, subdivided into sectors having angles of 107◦ , 156◦ , 29◦ and 68◦ , respectively

2. Mean 27.25, median 27, mode 26

11. (a) £495

3. Mean 4.7225, median 4.72, mode 4.72

(b) 88

4. Mean 115.2, median 126.4, no mode

12. (a) £16450 (b) 138

Exercise 113 (Page 237)

Exercise 111 (Page 233)

1. 23.85 kg 1. There is no unique solution, but one solution is: 39.3 − 39.4 39.9 − 40.0 40.5 − 40.6

1; 39.5 − 39.6 17; 40.1 − 40.2 4; 40.7 − 40.8

2. 171.7 cm

5; 39.7 − 39.8 9; 15; 40.3 − 40.4 2

7;

3. Mean 89.5, median 89, mode 89.2 4. Mean 2.02158 cm, median 2.02152 cm, mode 2.02167 cm

Answers to exercises

Exercise 114 (Page 239)

Exercise 120 (Page 250)

2. 2.83 µF

1. 4.60

1. 1

3. Mean 34.53 MPa, standard deviation 0.07474 MPa

4. 15x

4. 0.296 kg

7. 9

5. 9.394 cm

6. 2.828

2

2. 7

3. 8x

5. −4x + 3

6. 0

8.

2 3

9. 18x

10. −21x2 11. 2x + 15 Exercise 115 (Page 240) 1. 30, 27.5, 33.5 days

13. 12x2

4. 37 and 38; 40 and 41

5. 40, 40, 41; 50, 51, 51

Exercise 116 (Page 243) 2 or 0.2222 9

2. (a)

23 or 0.1655 139

(c)

69 or 0.4964 139

(b)

7 or 0.7778 9

(b)

1 (c) 36

2 4. (a) 5

1 (b) 5

4 (c) 15

(b)

1 200

(b) 0.2

2.

3√ t 2

3.

2 x

1 1 5. 3 + √ − 2 3 x 2 x

4. −

3 x4

6. −

10 7 + √ 3 x 2 x9

7. 6t − 12 8. 3x2 + 6x + 3

9. See answers for Exercise 120 (a) −15

(c)

9 1000

1. (a) 12 cos 3x (d)

1 50000

11. 6x2 + 6x − 4, 32

(b) 21

(b) −12 sin 6x

3. 270.2 A/s

2. 6 cos 3θ + 10 sin 2θ 4. 1393.4 V/s

5. 12 cos (4t + 0.12) + 6 sin (3t − 0.72) Exercise 123 (Page 254)

(c) 0.15 1. (a) 15e3x

2. (a) 0.64

1 √

Exercise 122 (Page 253) 13 (d) 15

Exercise 117 (Page 245) 1. (a) 0.6

1. 28x3

12. −6x2 + 4, −9.5

1 (b) 6

1 250

47 or 0.3381 139

Exercise 121 (Page 251)

10. 12x − 3

1 3. (a) 6

5. (a)

14. 6x

2. 27, 26, 33 faults

3. Q = 164.5 cm, Q = 172.5 cm, Q3 = 179 cm, 7.25 cm

1. (a)

12. 0

(b) −

(b) 0.32 3. 16

3. 0.0768

4 7e2x

2.

5 4 1 − = θ θ θ

4. 664

4. (a) 0.4912

(b) 0.4211

Exercise 124 (Page 255)

5. (a) 89.38%

(b) 10.25%

6. (a) 0.0227

(b) 0.0234

1. (a) −1 (b) 16 6 2 4 2. − 3 + + 10 sin 5x − 12 cos 2x + 3x x x e

(c) 0.0169

Exercise 125 (Page 255)

Exercise 118 (Page 247) 1. 1, 5, 21, 9, 61

2. 0, 11, −10, 21

3. proof

1. (a) 36x2 + 12x

(b) 72x + 12

1 4 12 6 − 5 + 3+ √ 5 t t 4 t3

4. 8, −a2 − a + 8, −a2 − a, −a − 1

2. (a)

Exercise 119 (Page 248)

3. −12 sin 2t − cos t

1. 16, 8

4. proof

(b) −4.95

283

284

Basic Engineering Mathematics

Exercise 126 (Page 256) 1. −2542 A/s

2. (a) 0.16 cd/V

(b) 312.5 V

3. (a) −1000 V/s (b) −367.9 V/s

(b)

2 3 x +c 2. (a) 15

7 2 x +c 2

(b) 3

x2 5x3 + 2 3

+ c or

3 2 x + 2 5x3 + c

4. (a)

3x2 − 5x + c 2

−4 +c 5. (a) 3x 6. (a)

2√ 3

x3 + c

7 (b) − cos 3x + c 3

8. (a)

3 2x e +c 8

−2 +c 15e5x

9. (a)

2 ln x + c 3

(b)

(b)

x2 − ln x + c 2

Exercise 128 (Page 261)

5 4 x +c (b) 24

4 3. (a) 3x + x2 − x3 + c 3

3 sin 2x + c 2

4. −1.635 Pa/m

Exercise 127 (Page 259) 1. (a) 4x + c

7. (a)

(b) 4x + 2x2 +

−1 (b) 3 + c 4x √ (b) 4 x + c

x3 +c 3

1. (a) 37.5

(b) 0.50

2. (a) 6

(b) −1.333

2. (a) 10.83

(b) −4

4. (a) 0

(b) 4

5. (a) 1

(b) 4.248

7. (a) 19.09

6. (a) 0.2352

(b) 2.457

(b) 2.638

8. (a) 0.2703

(b) 9.099

Exercise 129 (Page 264) 1. proof

2. proof

3. 32

6. 7.5 7. 1 8. 1.67

4. 29.33 Nm

9. 2.67

5. 37.5

10. 140 m

Index Abscissa, 83 Acute angle, 132, 143 Acute angled triangle, 134 Adding waveforms, 214 Addition of two periodic functions, 214 Algebra, 37 Algebraic expression, 37, 38, 41, 47 Alternate angles, 132, 147 Ambiguous case, 200 Amplitude, 156, 158 Angle, 131 Angle: lagging and leading, 157 types and properties of, 132 Angles of: any magnitude, 152 depression, 147 elevation, 147 Angular measurement, 131 Angular velocity, 158 Annulus, 169 Approximate value of calculations, 21, 22 Arbitrary constant of integration, 257 Arc, 174 Arc length, 175 Area under a curve, 261 Areas: irregular figures, 191, 263 plane figures, 166 sector, 167, 175 similar shapes, 172 triangles, 21, 167, 198 Arithmetic, basic, 1–4 Arithmetic progressions, 219 Average, 235 Average value of waveform, 194 Bar charts, 227 Bases, 14, 30 Binary numbers, 30–33 Blunder, 21, 22 BODMAS, 4, 7, 43 Boyle’s law, 45, 46 Brackets, 4, 41–43 Calculation of resultant phasors, 215–217 Calculations, 21–24 Calculator, 22–24, 103, 107, 111, 148, 164

Calculus, 247 Cancelling, 6, 7 Cartesian axes, 83 Cartesian co-ordinates, 162 Charle’s law, 45, 46 Chord, 174 Circle, 174 equation of, 178 properties of, 174 Circumference, 174 Classes, 230 Class: interval, 230 limits, 230 mid-point, 232 Coefficient of proportionality, 45 Common difference, 219 Common logarithms, 103 Common prefixes, 19, 20 ratio, 222 Complementary angles, 132 Completing the square, 71, 80 Computer numbering systems, 30 Cone, 180, 186 frustum of, 186 Congruent triangles, 136 Construction of triangles, 139, 140 Continuous data, 226 Conversion tables and charts, 25 Co-ordinates, 83 Corresponding angles, 132 Cosine, 143, 145, 148 graph of, 151, 154, 155 Cosine rule, 198 Cosine wave, 154, 155 Cubic equation, 100 Cuboid, 180 Cumulative frequency distribution, 230, 233 curve, 230 Cylinder, 180 Deciles, 239 Decimal: fraction, 9 places, 9–11 system, 9 Decimals, 9 Definite integrals, 260

286

Index

Degrees, 131 Denary system, 30, 33 Denominator, 6 Dependent event, 241 Dependent variable, 45 Depression, angle of, 147 Derivatives, 249 standard list, 254 Determination of law, 88, 117 involving logarithms, 119 Diameter, 174 Difference of two squares, 70 Differential calculus, 247 coefficient, 249 Differentiation, 247, 249 from first principles, 249 of axn , 250 of eax and ln ax, 253 of sine and cosine functions, 252 successive, 255 Digits, 9, 30 Direct proportion, 8, 45 Discrete data, 226 standard deviation, 237 Dividend, 38 Division, 1 Divisor, 38 Elevation, angle of, 147 Engineering notation, 19 Equation, 47 of circle, 178 Equations: cubic, 100 linear and quadratic, simultaneously, 75, 99 quadratic, 69, 76, 95 simple, 47 simultaneous, 60–68 Equilateral triangle, 134 Errors, 21 Evaluation of formulae, 21, 27 Expectation, 241 Exponent, 17–19 Exponential functions, 107 graphs of, 110 Expression, 47 Exterior angle of triangle, 134 Extrapolation, 89 Factorization, 41, 69, 80 Factors, 3 False axes, 89 Formula, 27, 72 Formulae: evaluation of, 21, 27 list of, 266–269 transposition of, 54–59 Fractions, 6 Frequency, 158, 226 Frequency distribution, 230, 231, 232 Frequency polygon, 230, 233

Frustum, 186 Full wave rectified waveform, 194 Functional notation, 247, 249 Geometric progressions, 222 Geometry, 131 Gradient, 83 of a curve, 248 Graphical solution of equations, 94 cubic, 100, 101 linear and quadratic, simultaneously, 75, 99 quadratic, 95–99 simultaneous, 94 Graphs, 83, 94 exponential functions, 110 logarithmic functions, 106 sine and cosine, 155 straight lines, 83 trigonometric functions, 151 with logarithmic scales, 124 of the form y = axn , 124 of the form y = abx , 127 of the form y = aekx , 128 Grouped data, 230 mean, median and mode, 236 standard deviation, 238 Growth and decay, laws of, 113 Half-wave rectified waveform, 194 Heptagon, 166 Hexadecimal number, 33–36 Hexagon, 166 Highest common factor (HCF), 3 Histogram, 230, 231, 233, 236 Hooke’s law, 45 Horizontal bar chart, 227 Hyperbolic logarithms, 103, 111 Hypotenuse, 142 Identity, 47 Improper fraction, 6 Indefinite integrals, 260 Independent event, 241 Independent variable, 45 Indices, 14 laws of, 14, 39, 265 Indicial equations, 105 Inequalities, 77 involving a modulus, 78 involving quotients, 79 involving square functions, 79 quadratic, 80 simple, 77, 78 Integers, 1 Integral calculus, 247 Integrals, 257 definite, 260 standard, 257 Integration, 247, 257 of axn , 257 Intercept, y-axis, 84, 86

Index

Interior angles, 132, 134 Interpolation, 89 Inverse proportion, 8, 45 Irregular areas, 191 Irregular volumes, 191, 192 Isosceles triangle, 134 Lagging angle, 157, 158 Laws of: algebra, 37 growth and decay, 113 indices, 14, 39 logarithms, 103 precedence, 4, 43 probability, 241 Leading angle, 157, 158 Leibniz notation, 249 Limiting value, 248 Linear and quadratic equations simultaneously, 99 Logarithmic functions, graphs of, 106 Logarithmic scales, 124 Logarithms, 103 laws of, 103 Log–linear graph paper, 127–129 Log–log graph paper, 124 Long division, 2, 10 Lower class boundary, 230 Lowest common multiple (LCM), 3, 49 Major arc, 174 Major sector, 174 Major segment, 174 Mantissa, 17 Maximum value, 95 Mean, 235 value of waveform, 191, 194 Measures of central tendency, 235 Median, 235, 238 Member of set, 226 Mensuration, 166 Mid-ordinate rule, 191, 264 Minimum value, 95 Minor arc, 174 Minor sector, 174 Minor segment, 174 Mixed number, 6 Mode, 235 Modulus, 78 Multiple, 3 Napierian logarithms, 103, 111 Natural logarithms, 103, 111 Nose-to-tail method, 208 Number sequences, 218 Numerator, 6 Obtuse angle, 132, 134 Obtuse angled triangle, 134 Octagon, 166, 171 Octal numbers, 32–34 Ogive, 230, 233, 239 Ohm’s law, 45

Order of magnitude error, 21, 22 Order of precedence, 4, 7 Ordinate, 83 Parabola, 95 Parallel lines, 132 Parallelogram, 166 method, 208 Pentagon, 166 Percentage component bar chart, 227 Percentage relative frequency, 226 Percentages, 6, 11 Percentile, 239 Perfect square, 70–72 Perimeter, 134 Period, 156 Periodic function, 155 Periodic time, 158 Phasor, 158, 214 Pictograms, 227 Pie diagram, 227, 229 Planimeter, 191 Plotting periodic functions, 214 Polar co-ordinates, 162 Polygon, 166 frequency, 230, 233 Population, 226 Power, 14, 17 series for ex , 108, 109 Practical problems: quadratic equations, 73–75 simple equations, 50–53 simultaneous equations, 65–68 straight line graphs, 88–91 trigonometry, 201–205 Precedence, 4, 43, 44 Prefixes, 19, 20 Prism, 180 Probability, 241 laws of, 241 Progression: arithmetic, 219 geometric, 222 Properties of triangles, 134 Proper fraction, 6, 10 Proportion, 8, 45 Pyramid, 183 volumes and surface area of frustum of, 186 Pythagoras’ theorem, 56, 142, 143, 162, 198 Quadrant, 174 Quadratic equations, 69 by completing the square, 69, 71, 72 factorization, 69 formula, 69, 72 practical problems, 73–75 Quadratic formula, 72 Quadratic graphs, 95 Quadratic inequalities, 80 Quadrilaterals, 166 properties of, 166

287

288

Index

Quartiles, 239 Quotients, inequalities involving, 79 Radians, 131, 158, 175 Radius, 174 Radix, 30 Range, 230 Ranking, 235 Rates of change, 254 Ratio and proportion, 8, 9 Reciprocal, 14 Rectangle, 166 Rectangular axes, 83, 152 co-ordinates, 164 Rectangular prism, 180 Reduction of non-linear laws to linear form, 117 Reflex angle, 132, 133 Relative frequency, 226 Relative velocity, 212 Resolution of vectors, 209 Resultant phasors, by calculation, 215 Rhombus, 166, 168 Right angle, 132 Right angled triangle, 134 solution of, 145 Rounding-off errors, 21 Sample, 226 Scalar quantities, 207 Scalene triangle, 134 Sector, 167, 174, 227 area of, 167, 175 Segment, 174 Semicircle, 167, 174 Semi-interquartile range, 239 Sequences, 218 Series, n’th term of, 218 Square numbers, 219 Set, 226, 230 Sequence of numbers, 218 Short division, 2 Significant figures, 9 Similar shapes, 172, 189 Similar triangles, 137, 138, 186 Simple equations, 47–51 practical problems, 50–53 Simple inequalities, 77, 78 Simpson’s rule, 192, 264 Simultaneous equations, 60–68, 94 practical problems, 65–68 Sine, 143, 145, 148 graph of, 151, 155 Sine rule, 198 wave, 154, 155, 194 Sinusoidal form A sin(ωt ± α), 158, 216 Slope, 83 Space diagram, 212 Sphere, 180 Square, 14, 166 root, 14, 49, 50, 56, 71

Square functions, inequalities involving, 79 Standard deviation, 237 Standard differentials, 254 integrals, 257 Standard form, 14, 17–19 Statistical data, presentation of, 226 Straight line graphs, 83 practical problems, 88–91 Subject of formulae, 27 Successive differentiation, 255 Sum to infinity of series, 222 Supplementary angles, 132 Surface areas of frusta of pyramids and cones, 186–189 of solids, 180–185 Symbols, 27 Tally diagram, 230–232 Tangent, 143, 148, 151, 174, 249, 253 Terminating decimal, 9 Term of series, 218 Transposition of formulae, 54–59 Transversal, 132 Trapezium, 166, 188 Trapezoidal rule, 191, 263 Triangle, 131, 134, 166 Triangles: area of, 21, 198 congruent, 136 construction of, 139, 140 properties of, 134 similar, 137, 138, 186 Trigonometric ratios, 143, 144 evaluation of, 148 graphs of, 151 Trigonometry, 142 practical situations, 201–205 Turning points, 95 Ungrouped data, 227 Upper class boundary, 230 Use of calculator, 22–24, 103, 107, 111, 148, 164 Vector addition, 207 Vector subtraction, 210 Vectors, 207 resolution of, 209 Vertical bar chart, 227 Vertically opposite angles, 132 Volumes of: common solids, 180 frusta of pyramids and cones, 186 irregular solids, 193 similar shapes, 189 Waveform addition, 214 y-axis intercept, 84 Young’s modulus of elasticity, 45, 90